Sensors for detecting analytes in fluids

ABSTRACT

Chemical sensors for detecting analytes in fluids comprise first and second conductive elements (e.g., electrical leads) electrically coupled to and separated by a chemically sensitive resistor which provides an electrical path between the conductive elements. The resistor comprises a plurality of alternating nonconductive regions (comprising a nonconductive organic polymer) and conductive regions (comprising a conductive material) transverse to the electrical path. The resistor provides a difference in resistance between the conductive elements when contacted with a fluid comprising a chemical analyte at a first concentration, than when contacted with a fluid comprising the chemical analyte at a second different concentration. Arrays of such sensors are constructed with at least two sensors having different chemically sensitive resistors providing dissimilar such differences in resistance. Variability in chemical sensitivity from sensor to sensor is provided by qualitatively or quantitatively varying the composition of the conductive and/or nonconductive regions. An electronic nose for detecting an analyte in a fluid may be constructed by using such arrays in conjunction with an electrical measuring device electrically connected to the conductive elements of each sensor.

The research carried out in the subject application was supported inpart by grants from the National Science Foundation (CHE 9202583), wasmade during the performance of work under a NASA contract, and issubject to the provisions of Public Law 96-517 (35 U.S.C. §202). Thegovernment may have rights in any patent issuing on this application.

INTRODUCTION

1. Field of the Invention

The field of the invention is electrical sensors for detecting analytesin fluids.

2. Background

There is considerable interest in developing sensors that act as analogsof the mammalian olfactory system (Lundstrom et al. (1991) Nature352:47-50; Shurmer and Gardner (1992) Sens. Act. B 8:1-11). This systemis thought to utilize probabilistic repertoires of many differentreceptors to recognize a single odorant (Reed (1992) Neuron 8:205-209;Lancet and Ben-Airie (1993) Curr. Biol. 3:668-674). In such aconfiguration, the burden of recognition is not on highly specificreceptors, as in the traditional "lock-and-key" molecular recognitionapproach to chemical sensing, but lies instead on the distributedpattern processing of the olfactory bulb and the brain (Kauer (1991)TINS 14:79-85; DeVries and Baylor (1993) Cell 10(S):139-149). Priorattempts to produce a broadly responsive sensor array have exploitedheated metal oxide thin film resistors (Gardner et al. (1991) Sens. Act.B 4:117-121; Gardner et al. (1991) Sens. Act. B 6:71-75; Corcoran et al.(1993) Sens. Act. B 15:32-37), polymer sorption layers on the surfacesof acoustic wave resonators (Grate and Abraham (1991) Sens. Act. B3:85-111; Grate et al. (1993) Anal. Chem. 65:1868-1881), arrays ofelectrochemical detectors (Stetter et al. (1986) Anal. Chem. 58:860-866;Stetter et al. (1990) Sens. Act. B 1:43-47; Stetter et al. (1993) Anal.Chem. Acta 284:1-11), or conductive polymers (Pearce et al. (1993)Analyst 118:371-377; Shurmer et al. (1991) Sens. Act. B 4:29-33). Arraysof metal oxide thin film resistors, typically based on SnO₂ films thathave been coated with various catalysts, yield distinct, diagnosticresponses for several vapors (Gardner et al. (1991) Sens. Act. B4:117-121; Gardner et al. (1991) Sens. Act. B 6:71-75; Corcoran et al.(1993) Sens. Act. B 15:32-37). However, due to the lack of understandingof catalyst function, SnO₂ arrays do not allow deliberate chemicalcontrol of the response of elements in the arrays nor reproducibility ofresponse from array to array. Surface acoustic wave resonators areextremely sensitive to both mass and acoustic impedance changes of thecoatings in array elements, but the signal transduction mechanisminvolves somewhat complicated electronics, requiring frequencymeasurement to 1 Hz while sustaining a 100 MHz Rayleigh wave in thecrystal (Grate and Abraham (1991) Sens. Act. B 3:85-111; Grate et al.(1993) Anal. Chem. 65:1868-1881). Attempts have been made to constructsensors with conducting polymer elements that have been grownelectrochemically through nominally identical polymer films and coatings(Pearce et al. (1993) Analyst 118:371-377; Shurmer et al. (1991) Sens.Act. B 4:29-33; Topart and Josowicz (1992) J. Phys. Chem. 96:7824-7830;Charlesworth et al. (1993) J. Phys. Chem. 97:5418-5423).

It is an object herein to provide a broadly responsive analyte detectionsensor based on one or more "chemiresistor" elements. Such elements aresimply prepared and are readily modified chemically to respond to abroad range of analytes. Such elements may also respond to temperatureand current variation. In addition, these sensors yield a rapid, lowpower, dc electrical signal in response to the fluid of interest, andtheir signals are readily integrated with software or hardware-basedneural networks for purposes of analyte identification.

Relevant Literature

Pearce et al. (1993) Analyst 118:371-377 and Gardner et al. (1994)Sensors and Actuators B 18-19:240-243 describe polypyrrole-based sensorarrays for monitoring beer flavor. Shurmer (1990) U.S. Pat. No.4,907,441, describes general sensor arrays with particular electricalcircuitry.

SUMMARY OF THE INVENTION

The invention provides methods, apparatuses and systems for detectingand identifying analytes in fluids. In one embodiment, the apparatusesinclude a chemical sensor comprising first and second conductiveelements (e.g., electrical leads) electrically coupled to a chemicallysensitive resistor which provides an electrical path between theconductive elements. The resistor comprises a plurality of alternatingnonconductive regions (comprising a nonconductive organic polymer) andconductive regions (comprising a conductive material). The electricalpath between the first and second conductive elements is transverse to(i.e., passes through) said plurality of alternating nonconductive andconductive regions. In use, the resistor provides a difference inresistance between the conductive elements when contacted with a fluidcomprising a chemical analyte at a first concentration, than whencontacted with a fluid comprising the chemical analyte at a seconddifferent concentration.

The electrical path through any given nonconductive region is typicallyon the order of 100 angstroms in length, providing a resistance of onthe order of 100 mΩ across the region. Variability in chemicalsensitivity from sensor to sensor is conveniently provided byqualitatively or quantitatively varying the composition of theconductive and/or nonconductive regions. For example, in one embodiment,the conductive material in each resistor is held constant (e.g., thesame conductive material such as polypyrrole) while the nonconductiveorganic polymer varies between resistors (e.g., different plastics suchas polystyrene).

Arrays of such sensors are constructed with at least two sensors havingdifferent chemically sensitive resistors providing dissimilardifferences in resistance. An electronic nose for detecting an analytein a fluid may be constructed by using such arrays in conjunction withan electrical measuring device electrically connected to the conductiveelements of each sensor. Such electronic noses may incorporate a varietyof additional components including means for monitoring the temporalresponse of each sensor, assembling and analyzing sensor data todetermine analyte identity, etc. Methods of making and using thedisclosed sensors, arrays and electronic noses are also provided.

In another embodiment, the sensor for detecting the presence of achemical analyte in a fluid comprises a chemically sensitive resistorelectrically connected to an electrical measuring apparatus where theresistor is in thermal communication with a temperature controlapparatus. The chemically sensitive resistor comprises a mixture of anonconductive organic polymer and a conductive material compositionallydifferent than said nonconductive organic polymer and provides anelectrical path therethrough. The chemically sensitive resistor providesvarying electrical resistances (R_(m)) at varying temperatures (T_(m))when contacted with a fluid comprising a particular chemical analyte.

Such sensors also function to provide an electrical impedance Z_(m) atfrequency ω_(m) when contacted with a fluid comprising a chemicalanalyte, where m is an integer greater than 1 and ω_(m) does not equal0. Finally, such sensors may also provide an electrical impedanceZ_(m),n at frequency ω_(m) and temperature T_(n) when contacted with afluid comprising a chemical analyte, where m and/or n is an integergreater than 1. The invention is also directed to systems and methodsfor employing such sensors for the detection of a chemical analyte in afluid.

BRIEF DESCRIPTION OF THE FIGURES

FIGS. 1A and 1A-1 show an overview of sensor design; FIG. 1B shows anoverview of sensor operation; FIG. 1C shows an overview of systemoperation.

FIG. 2 shows cyclic voltammogram of a poly(pyrrole)-coated platinumelectrode. The electrolyte was 0.10M (C₄ H₉)₄ N!⁺ ClO₄ !⁻ inacetonitrile, with a scan rate of 0.10 V s⁻¹.

FIG. 3A shows the optical spectrum of a spin coated poly(pyrrole) filmthat had been washed with methanol to remove excess pyrrole and reducedphosphomolybdic acid. FIG. 3B shows the optical spectrum of aspin-coated poly(pyrrole) film on indium-tin-oxide after 10 potentialcycles between +0.70 and -1.00 V vs. SCE in 0.10M (C₄ H₉)₄ N!⁺ ClO₄ !⁻in acetonitrile at a scan rate of 0.10 V -s⁻¹. The spectra were obtainedin 0.10M KCl--H₂ O.

FIGS. 4A and 4A-1 are is a schematic of a sensor array showing anenlargement of one of the modified ceramic capacitors used as sensingelements. The response patterns generated by the sensor array describedin Table 3 are displayed for: FIG. 4B acetone; FIG. 4C benzene; and FIG.4D ethanol.

FIG. 5 is a principle component analysis of autoscaled data fromindividual sensors containing different plasticizers. The numbers in theupper right hand corner of each square refer to the different sensorelements described in Table 3.

FIGS. 6A and 6B are principle component analysis of data obtained fromall sensors (Table 3). Conditions and symbols are identical to FIG. 5.FIG. 6A shows data represented in the first three principle componentspc1, pc2 and pc3, while FIG. 6B shows the data when represented in pc1,pc2, and pc4. A higher degree of discrimination between some solventscould be obtained by considering the fourth principle component asillustrated by larger separations between chloroform, tetrahydrofuran,and isopropyl alcohol in FIG. 6B.

FIG. 7A is a plot of acetone partial pressure (O) as a function of thefirst principle component; linear least square fit (--) between thepartial pressure of acetone and the first principle component (P_(a)=8.26•pc1+83.4, R² =0.989); acetone partial pressure (+) predicted froma multi-linear least square fit between the partial pressure of acetoneand the first three principle components (P_(a)=8.26•pc1-0.673•pc2+6.25•pc3+83.4, R² =0.998). FIG. 7B is a plot of themole fraction of methanol, x_(m), (O) in a methanol--ethanol mixture asa function of the first principle component; linear least square fit(------) between x_(m) and the first principle component (x_(m)=0.112•pc1+0.524, R² =0.979); x_(m) predicted from a multi-linear leastsquare fit (+) between x_(m) and the first three principle components(x_(m) =0.112•pc1-0.0300•pc2-0.0444•pc3+0.524, R² =0.987).

FIG. 8 is the resistance response of a poly(N-vinylpyrrolidone):carbonblack (20 w/w % carbon black) sensor element to methanol, acetone, andbenzene. The analyte was introduced at t=60 s for 60 s. Each trace isnormalized by the resistance of the sensor element (approx. 125Ω) beforeeach exposure.

FIG. 9 is the first three principal components for the response of acarbon-black based sensor array with 10 element. The non-conductivecomponents of the carbon-black composites used are listed in Table 3,and the resistors were 20 w/w % carbon black.

FIGS. 10A and 10B are is a schematic illustration of the sensor employedto measure the effect of temperature on differential resistanceresponses to various chemical analytes.

FIGS. 11A-11M are graphical representations of the data presented inTable 6 which is the average resistance measured by each sensor employedand for each of the eight analytes tested.

FIGS. 12A-12M are graphical representations of the data presented inTable 7 which is the normalized average resistance measured by eachsensor employed and for each of the either analytes tested.

DETAILED DESCRIPTION OF THE INVENTION

The invention provides sensor arrays for detecting an analyte in a fluidfor use in conjunction with an electrical measuring apparatus. Thesearrays comprise a plurality of compositionally different chemicalsensors. Each sensor comprises at least first and second conductiveleads electrically coupled to and separated by a chemically sensitiveresistor. The leads may be any convenient conductive material, usually ametal, and may be interdigitized to maximize signal-to-noise strength.

The resistor comprises a plurality of alternating nonconductive andconductive regions transverse to the electrical path between theconductive leads. Generally, the resistors are fabricated by blending aconductive material with a nonconductive organic polymer such that theelectrically conductive path between the leads coupled to the resistoris interrupted by gaps of non-conductive organic polymer material. Forexample, in a colloid, suspension or dispersion of particulateconductive material in a matrix of nonconductive organic polymermaterial, the matrix regions separating the particles provide the gaps.The nonconductive gaps range in path length from about 10 to 1,000angstroms, usually on the order of 100 angstroms providing individualresistance of about 10 to 1,000 mΩ, usually on the order of 100 mΩ,across each gap. The path length and resistance of a given gap is notconstant but rather is believed to change as the nonconductive organicpolymer of the region absorbs, adsorbs or imbibes an analyte.Accordingly, the dynamic aggregate resistance provided by these gaps ina given resistor is a function of analyte permeation of thenonconductive regions. In some embodiments, the conductive material mayalso contribute to the dynamic aggregate resistance as a function ofanalyte permeation (e.g., when the conductive material is a conductiveorganic polymer such as polyprryole).

A wide variety of conductive materials and nonconductive organic polymermaterials can be used. Table 1 provides exemplary conductive materialsfor use in resistor fabrication; mixtures, such as of those listed, mayalso be used. Table 2 provides exemplary nonconductive organic polymermaterials; blends and copolymers, such as of the polymers listed here,may also be used. Combinations, concentrations, blend stoichiometries,percolation thresholds, etc. are readily determined empirically byfabricating and screening prototype resistors (chemiresistors) asdescribed below.

                  TABLE 1                                                         ______________________________________                                        Major Class   Examples                                                        ______________________________________                                        Organic Conductors                                                                          conducting polymers (poly(anilines),                                          poly(thiophenes), poly(pyrroles),                                             poly(acetylenes), etc.)), carbonaceous                                        materials (carbon blacks, graphite, coke,                                     C.sub.60, etc.), charge transfer complexes                                    (tetramethylparaphenylenediamine-                                             chloranile, alkali metal                                                      tetracyanoquinodimethane complexes,                                           tetrathiofulvalene halide complexes, etc.),                                   etc.                                                            Inorganic Conductors                                                                        metals and metal alloys (Ag, Au, Cu, Pt,                                      AuCu alloy, etc.), highly doped                                               semiconductors (Si, GaAs, InP, MoS.sub.2,                                     TiO.sub.2, etc.), conductive metal oxides                                     (In.sub.2 O.sub.3, SnO.sub.2, Na.sub.x Pt.sub.3 O.sub.4,                      etc.),                                                                        superconductors (YBa.sub.2 Cu.sub.3 O.sub.7,                                  Tl.sub.2 Ba.sub.2 Ca.sub.2 Cu.sub.3 O.sub.10, etc.), etc.       Mixed inorganic/                                                                            Tetracyanoplatinate complexes, Iridium                          organic Conductors                                                                          halocarbonyl complexes, stacked                                               macrocyclic complexes, etc.                                     ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                        Major Class     Examples                                                      ______________________________________                                        Main-chain carbon polymers                                                                    poly(dienes), poly(alkenes),                                                  poly(acrylics), poly(methacrylics),                                           poly(vinyl ethers), poly(vinyl                                                thioethers), poly(vinyl alcohols),                                            poly(vinyl ketones), poly(vinyl                                               halides), poly(vinyl nitriles), poly(vinyl                                    esters), poly(styrenes), poly(arylenes),                                      etc.                                                          Main-chain acyclic heteroatom                                                                 poly(oxides), poly(carbonates),                               polymers        poly(esters), poly(anhydrides),                                               poly(urethanes), poly(sulfonates),                                            poly(siloxanes), poly(sulfides),                                              poly(thioesters), poly(sulfones),                                             poly(sulfonamides), poly(amides),                                             poly(ureas), poly(phosphazenes),                                              poly(silanes), poly(silazanes), etc.                          Main-chain      poly(furan tetracarboxylic acid                               heterocyclic polymers                                                                         diimides), poly(benzoxazoles),                                                poly(oxadiazoles),                                                            poly(benzothiazinophenothiazines),                                            poly(benzothiazoles),                                                         poly(pyrazinoquinoxalines),                                                   poly(pyromellitimides),                                                       poly(quinoxalines),                                                           poly(benzimidazoles), poly(oxindoles),                                        poly(oxoisoindolines),                                                        poly(dioxoisoindolines),                                                      poly(triazines), poly(pyridazines),                                           poly(piperazines), poly(pyridines),                                           poly(piperidines), poly(triazoles),                                           poly(pyrazoles), poly(pyrrolidines),                                          poly(carboranes),                                                             poly(oxabicyclononanes),                                                      poly(dibenzofurans), poly(phthalides),                                        poly(acetals), poly(anhydrides),                                              carbohydrates, etc.                                           ______________________________________                                    

The chemiresistors can be fabricated by many techniques such as, but notlimited to, solution casting, suspension casting, and mechanical mixing.In general, solution cast routes are advantageous because they providehomogeneous structures and ease of processing. With solution castroutes, resistor elements may be easily fabricated by spin, spray or dipcoating. Since all elements of the resistor must be soluble, however,solution cast routes are somewhat limited in their applicability.Suspension casting still provides the possibility of spin, spray or dipcoating but more heterogeneous structures than with solution casting areexpected. With mechanical mixing, there are no solubility restrictionssince it involves only the physical mixing of the resistor components,but device fabrication is more difficult since spin, spray and dipcoating are no longer possible. A more detailed discussion of each ofthese follows.

For systems where both the conducting and non-conducting media or theirreaction precursors are soluble in a common solvent, the chemiresistorscan be fabricated by solution casting. The oxidation of pyrrole byphosphomolybdic acid presented herein represents such a system. In thisreaction, the phosphomolybdic acid and pyrrole are dissolved intetrahydrofuran (THF) and polymerization occurs upon solventevaporation. This allows for THF soluble non-conductive polymers to bedissolved into this reaction mixture thereby allowing the blend to beformed in a single step upon solvent evaporation. The choice ofnon-conductive polymers in this route is, of course, limited to thosethat are soluble in the reaction media. For the poly(pyrrole) casedescribed above, preliminary reactions were performed in THF, but thisreaction should be generalizable to other non-aqueous solvent such asacetonitrile or ether. A variety of permutations on this scheme arepossible for other conducting polymers. Some of these are listed below.Certain conducting polymers, such as substitutedpoly(cyclooctatetraenes), are soluble in their undoped, non-conductingstate in solvents such as THF or acetonitrile. Consequently, the blendsbetween the undoped polymer and plasticizing polymer can be formed fromsolution casting. After which, the doping procedure (exposure to I₂vapor, for instance) can be performed on the blend to render thesubstituted poly(cyclooctatetraene) conductive. Again, the choice ofnon-conductive polymers is limited to those that are soluble in thesolvents that the undoped conducting polymer is soluble in and to thosestable to the doping reaction. Certain conducting polymers can also besynthesized via a soluble precursor polymer. In these cases, blendsbetween the precursor polymer and the non-conducting polymer can firstbe formed followed by chemical reaction to convert the precursor polymerinto the desired conducting polymer. For instance poly(p-phenylenevinylene) can be synthesized through a soluble sulfonium precursor.Blends between this sulfonium precursor and the non-conductive polymercan be formed by solution casting. After which, the blend can besubjected to thermal treatment under vacuum to convert the sulfoniumprecursor to the desired poly(p-phenylene vinylene).

In suspension casting, one or more of the components of the resistor issuspended and the others dissolved in a common solvent. Suspensioncasting is a rather general technique applicable to a wide range ofspecies, such as carbon blacks or colloidal metals, which can besuspended in solvents by vigorous mixing or sonication. In oneapplication of suspension casting, the non-conductive polymer isdissolved in an appropriate solvent (such as THF, acetonitrile, water,etc.). Colloidal silver is then suspended in this solution and theresulting mixture is used to dip coat electrodes.

Mechanical mixing is suitable for all of the conductive/non-conductivecombinations possible. In this technique, the materials are physicallymixed in a ball-mill or other mixing device. For instance, carbon black:non-conductive polymer composites are readily made by ball-milling. Whenthe non-conductive polymer can be melted or significantly softenedwithout decomposition, mechanical mixing at elevated temperature canimprove the mixing process.

Alternatively, composite fabrication can sometimes be improved byseveral sequential heat and mix steps.

Once fabricated, the individual elements can be optimized for aparticular application by varying their chemical make up andmorphologies. The chemical nature of the resistors determines to whichanalytes they will respond and their ability to distinguish differentanalytes. The relative ratio of conductive to insulating componentsdetermines the magnitude of the response since the resistance of theelements becomes more sensitive to sorbed molecules as the percolationthreshold is approached. The film morphology is also important indetermining response characteristics. For instance, thin films respondmore quickly to analytes than do thick ones. Hence, with an empiricalcatalogue of information on chemically diverse sensors made with varyingratios of insulating to conducting components and by differingfabrication routes, sensors can be chosen that are appropriate for theanalytes expected in a particular application, their concentrations, andthe desired response times. Further optimization can then be performedin an iterative fashion as feedback on the performance of an array underparticular conditions becomes available.

The resistor may itself form a substrate for attaching the lead or theresistor. For example, the structural rigidity of the resistors may beenhanced through a variety of techniques:chemical or radiationcross-linking of polymer components (dicumyl peroxide radicalcross-linking, UV-radiation cross-linking of poly(olefins), sulfurcross-linking of rubbers, e-beam cross-linking of Nylon, etc.), theincorporation of polymers or other materials into the resistors toenhance physical properties (for instance, the incorporation of a highmolecular weight, high transition metal (Tm) polymers), theincorporation of the resistor elements into supporting matrices such asclays or polymer networks (forming the resistor blends withinpoly-(methylmethacrylate) networks or within the lamellae ofmontmorillonite, for instance), etc. In another embodiment, the resistoris deposited as a surface layer on a solid matrix which provides meansfor supporting the leads. Typically, the matrix is a chemically inert,non-conductive substrate such as a glass or ceramic.

Sensor arrays particularly well-suited to scaled up production arefabricated using integrated circuit (IC) design technologies. Forexample, the chemiresistors can easily be integrated onto the front endof a simple amplifier interfaced to an A/D converter to efficiently feedthe data stream directly into a neural network software or hardwareanalysis section. Micro-fabrication techniques can integrate thechemiresistors directly onto a micro-chip which contains the circuitryfor analogue signal conditioning/processing and then data analysis. Thisprovides for the production of millions of incrementally differentsensor elements in a single manufacturing step using ink-jet technology.Controlled compositional gradients in the chemiresistor elements of asensor array can be induced in a method analogous to how a color ink-jetprinter deposits and mixes multiple colors. However, in this case ratherthan multiple colors, a plurality of different polymers in solutionwhich can be deposited are used. A sensor array of a million distinctelements only requires a 1 cm×1 cm sized chip employing lithography atthe 10 μm feature level, which is within the capacity of conventionalcommercial processing and deposition methods. This technology permitsthe production of sensitive, small-sized, stand-alone chemical sensors.

Preferred sensor arrays have a predetermined inter-sensor variation inthe structure or composition of the nonconductive organic polymerregions. The variation may be quantitative and/or qualitative. Forexample, the concentration of the nonconductive organic polymer in theblend can be varied across sensors. Alternatively, a variety ofdifferent organic polymers may be used in different sensors. Anelectronic nose for detecting an analyte in a fluid is fabricated byelectrically coupling the sensor leads of an array of compositionallydifferent sensors to an electrical measuring device. The device measureschanges in resistivity at each sensor of the array, preferablysimultaneously and preferably over time. Frequently, the device includessignal processing means and is used in conjunction with a computer anddata structure for comparing a given response profile to astructure-response profile database for qualitative and quantitativeanalysis. Typically such a nose comprises at least ten, usually at least100, and often at least 1000 different sensors though with massdeposition fabrication techniques described herein or otherwise known inthe art, arrays of on the order of at least 10⁶ sensors are readilyproduced.

In operation, each resistor provides a first electrical resistancebetween its conductive leads when the resistor is contacted with a firstfluid comprising a chemical analyte at a first concentration, and asecond electrical resistance between its conductive leads when theresistor is contacted with a second fluid comprising the same chemicalanalyte at a second different concentration. Moreover, a resistor mayprovide a first electrical resistance when the resistor is contactedwith a first fluid comprising a first chemical analyte at aconcentration C_(m) and a second electrical resistance when the resistoris contacted with a second fluid comprising a second, different chemicalanalyte at concentration C_(n), wherein C_(m) and C_(n) may be the sameor different. The fluids may be liquid or gaseous in nature. The firstand second fluids may reflect samples from two different environments, achange in the concentration of an analyte in a fluid sampled at two timepoints, a sample and a negative control, etc. The sensor arraynecessarily comprises sensors which respond differently to a change inan analyte concentration, i.e., the difference between the first andsecond electrical resistance of one sensor is different from thedifference between the first second electrical resistance of anothersensor.

In a preferred embodiment, the temporal response of each sensor(resistance as a function of time) is recorded. The temporal response ofeach sensor may be normalized to a maximum percent increase and percentdecrease in resistance which produces a response pattern associated withthe exposure of the analyte. By iterative profiling of known analytes, astructure-function database correlating analytes and response profilesis generated. Unknown analyte may then be characterized or identifiedusing response pattern comparison and recognition algorithms.Accordingly, analyte detection systems comprising sensor arrays, anelectrical measuring devise for detecting resistance across eachchemiresistor, a computer, a data structure of sensor array responseprofiles, and a comparison algorithm are provided. In anotherembodiment, the electrical measuring device is an integrated circuitcomprising neural network-based hardware and a digital-analog converter(DAC) multiplexed to each sensor, or a plurality of DACs, each connectedto different sensor(s).

A wide variety of analytes and fluids may be analyzed by the disclosedsensors, arrays and noses so long as the subject analyte is capablegenerating a differential response across a plurality of sensors of thearray. Analyte applications include broad ranges of chemical classessuch as organics such as alkanes, alkenes, alkynes, dienes, alicyclichydrocarbons, arenes, alcohols, ethers, ketones, aldehydes, carbonyls,carbanions, polynuclear aromatics and derivatives of such organics,e.g., halide derivatives, etc., biomolecules such as sugars, isoprenesand isoprenoids, fatty acids and derivatives, etc. Accordingly,commercial applications of the sensors, arrays and noses includeenvironmental toxicology and remediation, biomedicine, materials qualitycontrol, food and agricultural products monitoring, etc.

The general method for using the disclosed sensors, arrays andelectronic noses, for detecting the presence of an analyte in a fluidinvolves resistively sensing the presence of an analyte in a fluid witha chemical sensor comprising first and second conductive leadselectrically coupled to and separated by a chemically sensitive resistoras described above by measuring a first resistance between theconductive leads when the resistor is contacted with a first fluidcomprising an analyte at a first concentration and a second differentresistance when the resistor is contacted with a second fluid comprisingthe analyte at a second different concentration.

In another embodiment, the sensor for detecting the presence of achemical analyte in a fluid comprises a chemically sensitive resistorelectrically connected to an electrical measuring apparatus where theresistor is in thermal communication with a temperature controlapparatus. As described above, the chemically sensitive resistor(s)comprise a mixture of a nonconductive organic polymer and a conductivematerial which is compositionally different than the nonconductiveorganic polymer. The chemically sensitive resistor provides anelectrical path through which electrical current may flow and aresistance (R) at a temperature (T) when contacted with a fluidcomprising a chemical analyte.

In operation, the chemically sensitive resistor(s) of the sensor fordetecting the presence of a chemical analyte in a fluid provide anelectrical resistance (R_(m)) when contacted with a fluid comprising achemical analyte at a particular temperature (T_(m)). The electricalresistance observed may vary as the temperature varies, thereby allowingone to define a unique profile of electrical resistances at variousdifferent temperatures for any chemical analyte of interest. Forexample, a chemically sensitive resistor, when contacted with a fluidcomprising a chemical analyte of interest, may provide an electricalresistance R_(m) at temperature T_(m) where m is an integer greater than1, and may provide a different electrical resistance R_(n) at adifferent temperature T_(n). The difference between R_(m) and R_(n) isreadily detectable by an electrical measuring apparatus.

As such, the chemically sensitive resistor(s) of the sensor are inthermal communication with a temperature control apparatus, therebyallowing one to vary the temperature at which electrical resistances aremeasured. If the sensor comprises an array of two or more chemicallysensitive resistors each being in thermal communication with atemperature control apparatus, one may vary the temperature across theentire array (i.e., generate a temperature gradient across the array),thereby allowing electrical resistances to be measured simultaneously atvarious different temperatures and various different resistorcompositions. For example, in an array of chemically sensitiveresistors, one may vary the composition of the resistors in thehorizontal direction across the array, such that resistor composition inthe vertical direction across the array remains constant. One may thencreate a temperature gradient in the vertical direction across thearray, thereby allowing the simultaneous analysis of chemical analytesat different resistor compositions and different temperatures.

Methods for placing chemically sensitive resistors in thermalcommunication with a temperature control apparatus are readily apparentto those skilled in the art and include, for example, attaching aheating element to the sensor and passing electrical current throughsaid heating element. The temperature range across which electricalresistances may be measured will be a function of the resistorcomposition, for example the melting temperature of the resistorcomponents, the thermal stability of the analyte of interest or anyother component of the system, and the like. For the most part, thetemperature range across which electrical resistance will be measuredwill be about 20° C. to 80° C., preferably from about 22° C. to about70° C. and more preferably from about 22° C. to 65° C.

In yet another embodiment, rather than subjecting the sensor to a directelectrical current and measuring the true electrical resistance throughthe chemically sensitive resistor(s), the sensor can be subjected to analternating electrical current at different frequencies to measureimpedance. Impedance is the apparent resistance in an alternatingelectrical current as compared to the true electrical resistance in adirect current. As such, the present invention is also directed to asensor for detecting the presence of a chemical analyte in a fluid, saidsensor comprising a chemically sensitive resistor electrically connectedto an electrical measuring apparatus, said chemically sensitive resistorcomprising a mixture of nonconductive organic polymer and a conductivematerial compositionally different than said nonconductive organicpolymer and wherein said resistor provides (a) an electrical paththrough said mixture of nonconductive organic polymer and saidconductive material, and (b) an electrical impedance Z_(m) at frequencyω_(m) when contacted with a fluid comprising said chemical analyte,where m is an integer greater than 1 and ω_(m) does not equal 0. Formeasuring impedance as a function of frequency, the frequencies employedwill generally range from about 1 Hz to 5 GHz, usually from about 1 MHzto 1 GHz, more usually from about 1 MHz to 10 MHz and preferably fromabout 1 MHz to 5 MHz. Chemical analytes of interest will exhibit uniqueimpedance characteristics at varying alternating current frequencies,thereby allowing one to detect the presence of any chemical analyte ofinterest in a fluid by measuring Z_(m) at alternating frequency ω_(m).

For performing impedance measurements, one may employ virtually anyimpedance analyzer known in the art. Preferably, a Schlumberger Model1260 Impedance/Gain-Phase Analyzer (Schlumberger Technologies,Farnborough, Hampshire, England) with approximately 6 inch RG174 coaxialcables is employed. In such an apparatus, the resistor/sensor is held inan Al chassis box to shield them from external electronic noise.

In still another embodiment of the present invention, one may vary boththe frequency ω_(m) of the electrical current employed and thetemperature T_(n) and measure the electrical impedance Z_(m),n, therebyallowing for the detection of the presence of a chemical analyte ofinterest. As such, the present invention is also directed to a sensorfor detecting the presence of a chemical analyte in a fluid, said sensorcomprising a chemically sensitive resistor electrically connected to anelectrical measuring apparatus and being in thermal communication with atemperature control apparatus, said chemically sensitive resistorcomprising a mixture of nonconductive organic polymer and a conductivematerial compositionally different than said nonconductive organicpolymer, wherein said resistor provides (1) an electrical path throughsaid mixture of nonconductive organic polymer and said conductivematerial, and (2) an electrical impedance Z_(m),n at frequency ω_(m) andtemperature T_(n) when contacted with a fluid comprising said chemicalanalyte, where m and/or n is an integer greater than 1. For measuringimpedance as a function of frequency and temperature, the frequenciesemployed will generally not be higher than 10 MHz, preferably not higherthan 5 MHz. Chemical analytes of interest will exhibit unique impedancecharacteristics at varying alternating current frequencies and varyingtemperatures, thereby allowing one to detect the presence of anychemical analyte of interest in a fluid by measuring Z_(m),n atfrequency ω_(m) and temperature T_(n).

The present invention is also directed to systems and methods foremploying the above described sensors for detecting the presence of achemical analyte in a fluid.

The following examples are offered by way of illustration and not by wayof limitation.

EXAMPLES

I. Sensor Arrays

Polymer Synthesis

Poly(pyrrole) films used for conductivity, electrochemical, and opticalmeasurements were prepared by injecting equal volumes of N₂ -purgedsolutions of pyrrole (1.50 mmoles in 4.0 ml dry tetrahydrofuran) andphosphomolybdic acid (0.75 mmoles in 4.0 ml tetrahydrofuran) into a N₂-purged test tube. Once the two solutions were mixed, the yellowphosphomolybdic acid solution turned dark green, with no observableprecipitation for several hours. This solution was used for filmpreparation within an hour of mixing.

Sensor Fabrication

Plasticized poly(pyrrole) sensors were made by mixing two solutions, oneof which contained 0.29 mmoles pyrrole in 5.0 ml tetrahydrofuran, withthe other containing 0.25 mmoles phosphomolybdic acid and 30 mg ofplasticizer in 5.0 ml of tetrahydrofuran. The mixture of these twosolutions resulted in a w:w ratio of pyrrole to plasticizer of 2:3. Aninexpensive, quick method for crating the chemiresistor array elementswas accomplished by effecting a cross sectional cut through commercial22 nF ceramic capacitors (Kemet Electronics Corporation). Mechanicalslices through these capacitors revealed a series of interdigitatedmetal lines (25% Ag:75% Pt), separated by 15 μm, that could be readilycoated with conducting polymer. The monomer--plasticizer--oxidantsolutions were then used to dip coat interdigitated electrodes in orderto provide a robust electrical contact to the polymerized organic films.After polymerization was complete, the film was insoluble and was rinsedwith solvent (tetrahydrofuran or methanol) to remove residualphosphomolybdic acid and unreacted monomer. The sensors were thenconnected to a commercial bus strip, with the resistances of the various"chemiresistor" elements readily monitored by use of a multiplexingdigital ohmmeter.

Instrumentation

Optical spectra were obtained on a Hewlett Packard 8452Aspectrophotometer, interfaced to an IBM XT. Electrochemical experimentswere performed using a Princeton Applied Research Inc. 173potentiostat/175 universal programmer. All electrochemical experimentswere performed with a Pt flag auxiliary and a saturated calomelreference electrode (SCE). Spin-coating was performed on a HeadwayResearch Inc. photoresist spin coater. Film thicknesses were determinedwith a Dektak Model 3030 profilometer. Conductivity measurements wereperformed with an osmium-tipped four point probe (Alessi InstrumentsInc., tip spacing=0.050", tip radii=0.010"). Transient resistancemeasurements were made with a conventional multimeter (Fluke Inc.,"Hydra Data Logger" Meter).

Principle Component Analysis and Multi-linear Least Square Fits. A dataset obtained from a single exposure of the array to an odorant produceda set of descriptors (i.e., resistances), d_(i). The data obtained frommultiple exposures thus produced a data matrix D where each row,designated by j, consisted of n descriptors describing a single memberof the data set (i.e., a single exposure to an odor). Since the baselineresistance and the relative changes in resistance varied among sensor,sthe data matrix was autoscaled before further processing (Hecht (1990)Mathematics in Chemistry: An Introduction to Modern Methods (PrenticeHall, Englewood Cliffs, N.J.)). In this preprocessing technique, all thedata associated with a single descriptor (i.e., a column in the datamatrix) were centered around zero with unit standard deviation

    d'.sub.ij =(d.sub.ij -d.sub.i)/σ.sub.i               (1)

where d_(i) is the mean value for descriptor i and σ_(i) is thecorresponding standard deviation.

Principle component analysis (Hecht (1990)) was performed to determinelinear combinations of the data such that the maximum variance definedas the square of the standard deviation! between the members of the dataset was obtained in n mutually orthogonal dimensions. The linearcombinations of the data resulted in the largest variance or separation!between the members of the data set in the first principle component(pc1) and produced decreasing magnitudes of variance from the second tothe n^(th) principle component (pc2-pcn). The coefficients required totransform the autoscaled data into principle component space (by linearcombination) were determined by multiplying the data matrix, D, by itstranspose, D^(T) (i.e., diagnolizing the matrix) (Hecht (1990)Mathematics in Chemistry: An Introduction to Modern Methods (PrenticeHall, Englewood Cliffs, N.J.))

    R=D.sup.T •D                                         (2)

This operation produced the correlation matrix, R whose diagonalelements were unity and whose off-diagonal elements were the correlationcoefficients of the data. The total variance in the data was thus givenby the sum of the diagonal elements in R. The n eigenvalues, and thecorresponding n eigenvectors, were then determined for R. Eacheigenvector contained a set of n coefficients which were used totransform the data by linear combination into one of its n principlecomponents. The corresponding eigenvalue yielded the fraction of thetotal variance that was contained in that principle component. Thisoperation produced a principle component matrix, P, which had the samedimensions as the original data matrix. Under these conditions, each rowof the matrix P was still associated with a particular odor and eachcolumn was associated with a particular principle component.

Since the values in the principle component space had no physicalmeaning, it was useful to express the results of the principle componentanalysis in terms of physical parameters such as partial pressure andmole fraction. This was achieved via a multi-linear least square fitbetween the principle component values and the corresponding parameterof interest. A multi-linear least square fit resulted in a linearcombination of the principle components which yielded the best fit tothe corresponding parameter value. Fits were achieved by appending acolumn with each entry being unity to the principle component matrix P,with each row, j, corresponding to a different parameter value (e.g.,partial pressure), v_(j), contained in vector V. The coefficients forthe best multi-linear fit between the principle components and parameterof interest were obtained by the following matrix operation

    C=(P.sup.T •P).sup.-1 •P.sup.T •V        (3)

where C was a vector containing the coefficients for the linearcombination.

A key to our ability to fabricate chemically diverse sensing elementswas the preparation of processable, air stable films of electricallyconducting organic polymers. This was achieved through the controlledchemical oxidation of pyrrole (PY) using phosphomolybdic acid (H₃ PMo₁₂O₄₀) (20 in tetrahydrofuran:

    PY→PY.sup.•+ +e.sup.-                         (4)

    2PY.sup.•+ →PY.sub.2 +2H.sup.+                (5)

    H.sub.3 PMo.sub.12 O.sub.40 +2e.sup.- +2H.sup.+ →H.sub.5 PMo.sub.12 O.sub.40                                                  (6)

The redox-driven or electrochemically-induced polymerization of pyrrolehas been explored previously, but this process typically yieldsinsoluble, intractable deposits of poly(pyrrole) as the product (Salmonet al. (1982) J. Polym. Sci., Polym. Lett. 20:187-193). Our approach wasto use low concentrations of the H₃ PMo₁₂ O₄₀ oxidant (E^(o) =+0.36 Vvs. SCE) (Pope (1983) Heteropoly and Isopoly Oxometalates(Springer-Verlag, New York), Chap. 4). Since the electrochemicalpotential of PY⁺• /PY is more positive (E^(o) =+1.30 V vs. SCE)(Andrieux et al. (1990) J. Am. Chem. Soc. 112:2439-2440) than that of H₃PMo₁₂ O₄₀ /H₅ PMo₁₂ O₄₀, the equilibrium concentration of PY⁺ •, andthus the rate of polymerization, was relatively low in dilute solutions(0.19M PY, 0.09M H₃ PMo₁₂ O₄₀). However, it has been shown that theoxidation potential of pyrrole oligomers decreases from +1.20 V to +0.55to +0.26 V vs. SCE as the number of units increase from one to two tothree, and that the oxidation potential of bulk poly(pyrrole) occurs at-0.10 V vs. SCE (Diaz et al. (1981; ) J. Electroanal. Chem.121:355-361). As a result, oxidation of pyrrole trimers byphosphomolybdic acid is expected to be thermodynamically favorable. Thisallowed processing of the monomer-oxidant solution (i.e., spin coating,dip coating, introduction of plasticizers, etc.), after which timepolymerization to form thin films was simply effected by evaporation ofthe solvent. The dc electrical conductivity of poly(pyrrole) filmsformed by this method on glass slides, after rinsing the films withmethanol to remove excess phosphomolybdic acid and/or monomer, was onthe order of 15-30 S-cm⁻¹ for films ranging from 40-100 nm in thickness.

The poly(pyrrole) films produced in this work exhibited excellentelectrochemical and optical properties. For example, FIG. 2 shows thecyclic voltammetric behavior of a chemically polymerized poly(pyrrole)film following ten cycles from -1.00 V to +0.70 V vs. SCE. The cathodicwave at -0.40 V corresponded to the reduction of poly(pyrrole) to itsneutral, nonconducting state, and the anodic wave at -0.20 Vcorresponded to the reoxidation of poly(pyrrole) to its conducting state(Kanazawa et al. (1981) Synth. Met. 4:119-130). The lack of additionalfaradaic current, which would result from the oxidation and reduction ofphosphomolybdic acid in the film, suggests that the Keggin structure ofphosphomolybdic acid was not present in the film anions (Bidan et al.(1988) J. Electroanal. Chem. 251:297-306) and implies that MoO₄ ²⁻, orother anions, served as the poly(pyrrole) counterions in the polymerizedfilms.

FIG. 3A shows the optical spectrum of a processed polypyrrole film thathad been spin-coated on glass and then rinsed with methanol. The singleabsorption maximum was characteristic of a highly oxidized poly(pyrrole)(Kaufman et al. (1984) Phys. Rev. Lett. 53:1005-1008), and theabsorption band at 4.0 eV was characteristic of an interband transitionbetween the conduction and valence bands. The lack of other bands inthis energy range was evidence for the presence of bipolaron states (seeFIG. 3A), as have been observed in highly oxidized poly(pyrrole) (Id.).By cycling the film in 0.10M (C₄ H₉)₄ N!⁺ ClO₄ !⁻ --acetonitrile andthen recording the optical spectra in 0.10M KCl--H₂ O, it was possibleto observe optical transitions characteristic of polaron states inoxidized poly(pyrrole) (see FIG. 3B). The polaron states have beenreported to produce three optical transitions (Id.), which were observedat 2.0, 2.9, and 4.1 eV in FIG. 3B. Upon reduction of the film (c.f.FIG. 3B), an increased intensity and a blue shift in the 2.9 eV band wasobserved, as expected for the π→π* transition associated with thepyrrole units contained in the polymer backbone (Yakushi et al. (1983)J. Chem. Phys. 79:4774-4778).

As described in the experimental section, various plasticizers wereintroduced into the polymer films (Table 3).

                  TABLE 3                                                         ______________________________________                                        Plasticizers used in array elements*                                          sensor         plasticizer                                                    ______________________________________                                        1              none                                                           2              none**                                                         3              poly(styrene)                                                  4              poly(styrene)                                                  5              poly(styrene)                                                  6              poly(a-methyl styrene)                                         7              poly(styrene-acrylonitrile)                                    8              poly(styrene-maleic anydride)                                  9              poly(styrene-allyl alcohol)                                    10             poly(vinyl pyrrolidone)                                        11             poly(vinyl phenol)                                             12             poly(vinyl butral)                                             13             poly(vinyl acetate)                                            14             poly(carbonate)                                                ______________________________________                                         *Sensors contained 2:3 (w:w) ratio of pyrrole to plasticizer.                 **Film not rinsed to remove excess phosphomolybdic acid.                 

These inclusions allowed chemical control over the binding propertiesand electrical conductivity of the resulting plasticized polymers.Sensor arrays consisted of as many as 14 different elements, with eachelement synthesized to produce a distinct chemical composition, and thusa distinct sensor response, for its polymer film. The resistance, R, ofeach film-coated individual sensor was automatically recorded before,during, and after exposure to various odorants. A typical trialconsisted of a 60 sec rest period in which the sensors were exposed toflowing air (3.0 liter-min⁻¹), a 60 sec exposure to a mixture of air(3.0 liter-min⁻¹) and air that had been saturated with solvent (0.5-3.5liter-min⁻¹), and then a 240 sec exposure to air (3.0 liter-min⁻¹).

In an initial processing of the data, presented in this paper, the onlyinformation used was the maximum amplitude of the resistance changedivided by the initial resistance, ΔR_(max) /R_(i), of each individualsensor element. Most of the sensors exhibited either increases ordecreases in resistance upon exposure to different vapors, as expectedfrom changes in the polymer properties upon exposure to different typeschemicals (Topart and Josowicz (1992) J. Phys. Chem. 96:7824-7830;Charlesworth et al. (1993) J. Phys. Chem. 97:5418-5423). However, insome cases, sensors displayed an initial decrease followed by anincrease in resistance in response to a test odor. Since the resistanceof each sensor could increase and/or decrease relative to its initialvalue, two values of ΔR_(max) /R_(i) were reported for each sensor. Thesource of the bi-directional behavior of some sensor/odor pairs has notyet been studied in detail, but in most cases this behavior arose fromthe presence of water (which by itself induced rapid decreases in thefilm resistance) in the reagent-grade solvents used to generate the testodors of this study. The observed behavior in response to theseair-exposed, water-containing test solvents was reproducible andreversible on a given sensor array, and the environment wasrepresentative of many practical odor sensing applications in which airand water would not be readily excluded.

FIGS. 4B-4D depict representative examples of sensor amplitude responsesof a sensor array (see, Table 3). In this experiment, data were recordedfor three separate exposures to vapors of acetone, benzene, and ethanolflowing in air. The response patterns generated by the sensor arraydescribed in Table 3 are displayed for: (B) acetone; (C) benzene; and(D) ethanol. The sensor response was defined as the maximum percentincrease and decrease of the resistance divided by the initialresistance (gray bar and black bar, respectively) of each sensor uponexposure to solvent vapor. In many cases sensors exhibited reproducibleincreases and decreases in resistance. An exposure consisted of: (i) a60 sec rest period in which the sensors were exposed to flowing air (3.0liter-min⁻¹); (ii) a 60 sec exposure to a mixture of air (3.0liter-min⁻¹) and air that had been saturated with solvent (0.5liter-min⁻¹); and (iii) a 240 sec exposure to air (3.0 liter-min⁻¹). Itis readily apparent that these odorants each produced a distinctiveresponse on the sensor array. In additional experiments, a total of 8separate vapors (acetone, benzene, chloroform, ethanol, isopropylalcohol, methanol, tetrahydrofuran, and ethyl acetate), chosen to span arange of chemical and physical characteristics, were evaluated over afive-day period on a 14-element sensor array (Table 3). As discussedbelow, each odorant could be clearly and reproducibly identified fromthe others using this sensor apparatus.

Principle component analysis (Hecht (1990) Mathematics in Chemistry: AnIntroduction to Modern Methods (Prentice Hall, Englewood Cliffs, N.J.))was used to simplify presentation of the data and to quantify thedistinguishing abilities of individual sensors and of the array as awhole. In this approach, linear combinations of the ΔR_(max) /R_(i) datafor the elements in the array were constructed such that the maximumvariance (defined as the square of the standard deviation) was containedin the fewest mutually orthogonal dimensions. This allowedrepresentation of most of the information contained in data sets shownin FIGS. 4B-4D in two (or three) dimensions. The resulting clustering,or lack thereof, of like exposure data in the new dimensional space wasused as a measure of the distinguishing ability, and of thereproducibility, of the sensor array.

In order to illustrate the variation in sensor response of individualsensors that resulted from changes in the plasticizing polymer,principle component analysis was performed on the individual, isolatedresponses of each of the 14 individual sensor elements in a typicalarray (FIG. 5). Data were obtained from multiple exposures to acetone(a), benzene (b), chloroform (c), ethanol (e), isopropyl alcohol (i),methanol (m), tetrahydrofuran (t), or ethyl acetate (@) over a period offive days with the test vapors exposed to the array in varioussequences. The numbers of the figures refer to the sensor elementsdescribed in Table 3. The units along the axes indicate the amplitude ofthe principle component that was used to describe the particular dataset for an odor. The black regions indicate clusters corresponding to asingle solvent which could be distinguished from all others; grayregions highlight data of solvents whose signals overlapped with othersaround it. Exposure conditions were identical to those in FIG. 4.

Since each individual sensor produced two data values, principlecomponent analysis of these responses resulted in only two orthogonalprincipal components; pc1 and pc2. As an example of the selectivityexhibited by an individual sensor element, the sensor designated asnumber 5 in FIG. 5 (which was plasticized with poly(styrene)) confusedacetone with chloroform, isopropyl alcohol, and tetrahydrofuran. It alsoconfused benzene with ethyl acetate, while easily distinguishing ethanoland methanol from all other solvents. Changing the plasticizer to poly(α-methyl styrene) (sensor number 6 in FIG. 5) had little effect on thespatial distribution of the responses with respect to one another andwith respect to the origin. Thus, as expected, a rather slight chemicalmodification of the plasticizer had little effect on the relativevariance of the eight test odorants. In contrast, the addition of acyano group to the plasticizer, in the form ofpoly(styrene-acrylonitrile), (sensor number 7 in FIG. 5), resulted in alarger contribution to the overall variance by benzene and chloroform,while decreasing the contribution of ethanol. Changing the substituentgroup in the plasticizer to a hydrogen bonding acid (poly(styrene-allylalcohol), sensor number 9 in FIG. 5) increased the contribution ofacetone to the overall variance while having little effect on the otherodors, with the exception of confusing methanol and ethanol. Theseresults suggest that the behavior of the sensors can be systematicallyaltered by varying the chemical composition of the plasticizing polymer.

FIGS. 6A and 6B show the principle component analysis for all 14 sensorsdescribed in Table 3 and FIGS. 4 and 5. When the solvents were projectedinto a three dimensional odor space (FIG. 6A or 6B), all eight solventswere easily distinguished with the specific array discussed herein.Detection of an individual test odor, based only on the criterion ofobserving ˜1% ΔR_(max) /R_(i) values for all elements in the array, wasreadily accomplished at the parts per thousand level with no controlover the temperature or humidity of the flowing air. Further increasesin sensitivity are likely after a thorough utilization of the temporalcomponents of the ΔR_(max) /R_(i) data as well as a more completecharacterization of the noise in the array.

We have also investigated the suitability of this sensor array foridentifying the components of certain test mixtures. This task isgreatly simplified if the array exhibits a predictable signal responseas the concentration of a given odorant is varied, and if the responsesof various individual odors are additive (i.e., if superposition ismaintained). When a 19-element sensor array was exposed to a number, n,of different acetone concentrations in air, the (CH₃)₂ CO concentrationwas semi-quantitavely predicted from the first principle component. Thiswas evident from a good linear least square fit through the first threeprinciple components (see FIG. 7A for the linear least square fit forthe first principle component).

The same sensor array was also able to resolve the components in varioustest methanol-ethanol mixtures (Morris et al. (1942) Can. J. Res. B20:207-211. As shown in FIG. 7B, a linear relationship was observedbetween the first principle component and the mole fraction of methanolin the liquid phase, x_(m), in a CH₃ OH-C₂ H₅ OH mixture, demonstratingthat superposition held for this mixture/sensor array combination.Furthermore, although the components in the mixture could be predictedfairly accurately from just the first principle component, an increasein the accuracy could be achieved using a multi-linear least square fitthrough the first three principle components. This relationship held forCH₃ OH/(CH₃ OH+C₂ H₅ OH) ratios of 0 to 1.0 in air-saturated solutionsof this vapor mixture. The conducting polymer-based sensor arrays couldtherefore not only distinguish between pure test vapors, but alsoallowed analysis of concentrations of odorants as well as analysis ofbinary mixtures of vapors.

In summary, the results presented herein advance the area of analytesensor design. A relatively simple array design, using only amultiplexed low-power dc electrical resistance readout signal, has beenshown to readily distinguish between various test odorants. Suchconducting polymer-based arrays are simple to construct and modify, andafford an opportunity to effect chemical control over the responsepattern of a vapor. For example, by increasing the ratio of plasticizerto conducting polymer, it is possible to approach the percolationthreshold, at which point the conductivity exhibits a very sensitiveresponse to the presence of the sorbed molecules. Furthermore, producingthinner films will afford the opportunity to obtain decreased responsetimes, and increasing the number of plasticizing polymers and polymerbackbone motifs will likely result in increased diversity among sensors.This type of polymer-based array is chemically flexible, is simple tofabricate, modify, and analyze, and utilizes a low power dc resistancereadout signal transduction path to convert chemical data intoelectrical signals. It provides a new approach to broadly-responsiveodor sensors for fundamental and applied investigations of chemicalmimics for the mammalian sense of smell. Such systems are useful forevaluating the generality of neural network algorithms developed tounderstand how the mammalian olfactory system identifies thedirectionality, concentration, and identity of various odors.

Fabrication and Testing of Carbon Black-based Sensor Arrays

Sensor Fabrication

Individual sensor elements were fabricated in the following manner. Eachnon-conductive polymer (80 mg, see Table 4) was dissolved in 6 ml ofTHF.

                  TABLE 4                                                         ______________________________________                                        Sensor #      Non-Conductive Polymer                                          ______________________________________                                        1             poly(4-vinyl phenol)                                            2             poly(styrene - allyl alcohol)                                   3             poly(α-methyl styrene)                                    4             poly(vinyl chloride - vinyl acetate)                            5             poly(vinyl acetate)                                             6             poly(N-vinyl pyrrolidone)                                       7             poly(bisphenol A carbonate)                                     8             poly(styrene)                                                   9             poly(styrene-maleic anhydride)                                  10            poly(sulfone)                                                   ______________________________________                                    

Then, 20 mg of carbon black (BP 2000, Cabot Corp.) were suspended withvigorous mixing. Interdigitated electrodes (the cleaved capacitorspreviously described) were then dipped into this mixture and the solventallowed to evaporate. A series of such sensor elements with differingnon-conductive polymers were fabricated and incorporated into acommercial bus strip which allowed the chemiresistors t o be easilymonitored with a multiplexing ohmmeter.

Sensor Array Testing

To evaluate the performance of the carbon-black based sensors, arrayswith as many as 20 elements were exposed to a series of analytes. Asensor exposure consisted of (1) a 60 second exposure to flowing air (6liter min-1), (2) a 60 second exposure to a mixture of air (6 litermin-1) and air that had been saturated with the analyte (0.5 litermin-1), (3) a five minute recovery period during which the sensor arraywas exposed to flowing air (6 liter min-1). The resistance of theelements were monitored during exposure, and depending on the thicknessand chemical make-up of the film, resistance changes as large as 250%could be observed in response to an analyte. In one experiment, a 10element sensor array consisting carbon-black composites formed with aseries of non-conductive polymers (see Table 4) was exposed to acetone,benzene, chloroform, ethanol, hexane, methanol, and toluene over a twoday period. A total of 58 exposures to these analytes were performed inthis time period. In all cases,resistance changes in response to theanalytes were positive,and with the exception of acetone, reversible(see FIG. 8). The maximum positive deviations were then subjected toprincipal component analysis in a manner analogous to that described forthe poly(pyrrole) based sensor. FIG. 9 shows the results of theprincipal component analysis for the entire 10-element array. With theexception of overlap between toluene with benzene, the analytes weredistinguished from one and other.

II. Temperature Variance (Experiment 1)

Sensor Fabrication

Into 15 ml of benzene, was dissolved 160 mg of polymer (eitherpoly(styrene) and poly(ethylene-co-vinyl acetate) and to this was added40 mg of carbon black forming a suspension. The sensor substrate was a10 mm by 25 mm glass slide onto which had been evaporated two gold padsapproximately 1000 angstroms thick. These gold pads entirely covered oneface of the slide with the exception of a 5 mm wide section across themiddle of one face of the slide. The glass slide was rapidly dipped intothe polymer suspension of carbon black 5-10 times to coat the slide withthe polymer composite. The face opposite the gold pads was wiped cleanof deposits.

The heating element was made from 24 gauge nickle chromium wire whichwas bent into a zig-zag shape with each turn being equal in length tothe width of the sensor element. Wire tails were allowed to extend about1 cm from opposite sides of the sensors for external electricalconnections. The total length of the heating elements was approximately7 cm. The heating element was attached to the back (the face oppositethe gold pads) with epoxy (able to withstand 120° C.) by sandwiching itbetween the sensor and another (uncoated) glass slide of the same sizeas the sensor. The final sensor configuration was allowed to stand 5-10days before use. The heating elements were heated by passing 0.8, 1.2 or1.6 A of current through the elements.

Data Collection

Each sensor was placed in the flow chamber individually, with allelectrical connections in place, and allowed to equilibrate to thebackground air flow until a stable baseline was achieved. Each exposureconsisted of taking data for 60 seconds to establish a baseline,followed by a 60 second exposure to the analyte vapor stream and finallyfollowed by a five minute recovery period before the next cycle began.For both sensors, data were collected for exposures to chloroform andbenzene at four different temperatures for each solvent. First, datawere collected for analyte exposure while at ambient temperature (about22° C.; i.e., "temperature 1"), followed by the temperature associatedwith passing 0.8 A of electrical current through the heating element("temperature 2"), then 1.2 A ("temperature 3") and then 1.6 A ofcurrent ("temperature 4") (the latter temperatures being undetermined).The sensors were allowed to equilibrate at each new temperature for 15minutes before data collection was initiated. Three exposures at eachcurrent were taken.

Data Analysis

The data streams were saved to the computer during the exposures to theanalyte. Baseline resistances were taken by averaging the resistancedata during the 20 seconds prior to exposure to the analyte. The maximumresistance was extracted from the data stream and the change inresistance between baseline and maximum resistance was calculated (ΔR).This ΔR value was divided by the baseline resistance (R) and multipliedby 100 to express the response as a percentage. The percent responsesfor the three exposures at each temperature were averaged to obtain onevalue for each temperature/solvent combination. The responses for eachvapor at the four temperatures tested were normalized by summing thefour values and dividing each response by this sum. The results of theseexperiments are presented in Table 5.

                  TABLE 5                                                         ______________________________________                                        Polymer composition/analyte                                                                     (ΔR/R) × 100                                                                 Normalized                                       ______________________________________                                        A. Poly(styrene)/Chloroform                                                   temperature 1     0.60770905 0.23762355                                       temperature 2     0.69325945 0.27107507                                       temperature 3     0.75494462 0.29519491                                       temperature 4     0.50153141 0.19610647                                       Sum               2.55744454                                                  B. Poly(styrene)/Benzene                                                      temperature 1     0.33986648 0.38334474                                       temperature 2     0.23028222 0.25974165                                       temperature 3     0.14179893 0.15993891                                       temperature 4     0.17463419 0.1969747                                        Sum               0.88658181                                                  C. Poly(ethylene-co-vinyl acetate)/Chloroform                                 temperature 1     33.5395398 0.63563285                                       temperature 2     12.1030896 0.22937468                                       temperature 3      5.71049748                                                                              0.1082239                                        temperature 4      1.41245945                                                                              0.02676857                                       Sum               52.7655863                                                  D. Poly(ethylene-co-vinyl acetate)/Benzene                                    temperature 1     10.236522  0.54522797                                       temperature 2      5.66926122                                                                              0.30196192                                       temperature 3     2.37495836 0.12649743                                       temperature 4     0.49401403 0.02631267                                       Sum               18.7747556                                                  ______________________________________                                    

The results in Table 5 demonstrate that both the poly(styrene) andpoly(ethylene-co-vinyl acetate) sensors provided different electricalresistances at different temperatures with the two analytes tested.Moreover, the results presented in Table 5 demonstrate that differentanalytes provide different, unique patterns of electrical resistance atthe various temperatures tested. Therefore, these data demonstrate thatby varying the temperature at which the electrical resistance readingsare obtained from the sensor element, one is able to identify a uniquepattern of electrical resistances for any analyte of interest, therebyallowing for the identification of an analyte of interest.

III. Temperature Variance (Experiment 2)

Three sensor composites were used. These were poly(ethylene-co-vinylacetate), poly(ethylene oxide), and poly(4-vinyl phenol). Thesecomposites were blended with conductive carbon black in a 1:4 carbonblack to polymer ratio. The composite material was coated onto a glassslide which had evaporated gold tabs as electrical contacts. To theunderside of the glass slide (the side without the polymer film) wasaffixed nickel-chromium heating wire using low vapor pressure/hightemperature epoxy. The wire was bent into a zig zag shape and affixed sothat the main heating area was directly opposite the region between thegold tabs. Next was attached the underside of the heating wire a plain(not coated with gold or polymer) glass slide of the same dimension asthe substrate slide. In this configuration the heating wire wassandwiched between two glass slides. A thin thermocouple was glued tothe second glass slide on the opposite side from the heating wire.Therefore, the temperature detection was made at a similar orientationto the heater as the polymer composite film. (See FIG. 10).

The current for the resistive heaters was supplied by one common powersupply. The resistive heaters were wired in parallel with the powersupply. There was a rheostat (variable resistor) in series with eachbranch of the parallel circuit. This way the amount of current to eachheater, and subsequently its temperature, could be controlled. Thetemperatures were measured by the attached thermocouples. Thesetemperatures were not monitored continuously, but were recorded beforean experiment began and at selected times during the experiment. Onceequilibrium was reached, the temperature of the sensor/heater did notdeviate more than 0.1° C. over the course of minutes and did not deviatemore than 1° C. over the course of each experiment (about 3 hours).

Multiple copies of each sensor/heater were made and placed into a flowchamber. For poly(ethylene oxide), five sensors were made; sensors 4, 6,9, 15, and 16. Sensors 15 and 16 were heated, with sensor 15 being setto higher temperatures. Sensors 4, 6, and 9 were not heated. Forpoly(ethylene--co--vinyl acetate), four sensors were made; sensors 3, 8,17, and 20. Sensors 17 and 20 were heated, with sensor 17 being set tohigher temperatures. Sensors 3 and 8 were not heated. For poly(4-vinylphenol), four sensors were made; sensors 5, 7, 18, and 19. Sensors 18and 19 were heated, with sensor 18 being set to higher temperatures.Sensors 5 and 7 were not heated.

In these experiments, exposures were made to eight different solvents:benzene, chloroform, toluene, cyclohexane, hexane, 2-propanol, ethanol,and methanol. Each solvent was exposed at a concentration of 900 ppm. Inevery experiment, the solvents were exposed to the sensors three timesin the order listed above. The average of these three trials was takenand the standard deviation calculated.

Six experiments were performed. First, all the sensors were kept at roomtemperature during the exposures. Second, the heated sensors were heatedto the first set of temperature values, TEMP1. (The specific temperaturevalues are listed in Tables 6 and 7 below.) This was followed by anotherexposure set where all the sensors were at room temperature. Fourth, thesecond temperature experiment was repeated, TEMP2. The temperatures ofTEMP1 and of TEMP2 were meant to be nominally the same values (see datatables below). The fifth experiment was another set of room temperatureexposures. Finally, another heated exposure set, TEMP3, was collected.

The temperatures of the sensors in TEMP3 were adjusted to be higher thanthose in TEMP1 or TEMP2.

The average resistance and the standard deviations for the three trialsin each experiment were calculated and are listed in Table 6. Since theover-all signal for each sensor decreases at higher temperatures, theaverage responses and the standard deviation for each sensor werenormalized and are listed in Table 7. The first, second, and thirdcolumns of the tables are the three room temperature experiments. Thelast three columns are TEMP1, TEMP2, and TEMP3, respectively. The valuesin Tables 6 and 7 are also presented graphically in FIGS. 11 and 12.

                                      TABLE 6                                     __________________________________________________________________________    Exp 1      Exp 3 Exp 5                                                                              Exp 2 Exp 4                                                                              Exp 6                                        __________________________________________________________________________    22° C.                                                                            22° C.                                                                       23° C.                                                                      22° C.                                                                       22° C.                                                                      22° C.                                __________________________________________________________________________    AVERAGES                                                                      Sensor 3                                                                      poly(ethylene-vinyl acetate) (not heated)                                     2-propanol                                                                          1.81E-03                                                                           1.75E-03                                                                            1.91E-03                                                                           1.84E-03                                                                            1.94E-03                                                                           1.84E-03                                     benzene                                                                             6.25E-03                                                                           6.07E-03                                                                            6.52E-03                                                                           6.54E-03                                                                            6.49E-03                                                                           6.19E-03                                     chloroform                                                                          5.67E-03                                                                           5.36E-03                                                                            5.92E-03                                                                           5.96E-03                                                                            5.81E-03                                                                           5.57E-03                                     cyclohexane                                                                         5.08E-03                                                                           4.89E-03                                                                            5.35E-03                                                                           5.17E-03                                                                            5.27E-03                                                                           5.06E-03                                     ethanol                                                                             1.33E-03                                                                           1.31E-03                                                                            1.40E-03                                                                           1.40E-03                                                                            1.39E-03                                                                           1.33E-03                                     hexane                                                                              2.95E-03                                                                           2.67E-03                                                                            2.90E-03                                                                           3.04E-03                                                                            2.93E-03                                                                           2.75E-03                                     methanol                                                                            3.31E-04                                                                           3.21E-04                                                                            3.53E-04                                                                           4.27E-04                                                                            3.76E-04                                                                           3.24E-04                                     toluene                                                                             2.59E-02                                                                           2.51E-02                                                                            2.69E-02                                                                           2.63E-02                                                                            2.68E-02                                                                           2.50E-02                                     STANDARD DEVIATION                                                            Sensor 3                                                                      poly(ethylene-vinyl acetate) (not heated)                                     2-propanol                                                                          1.31E-04                                                                           3.54E-05                                                                            1.73E-05                                                                           1.15E-04                                                                            4.06E-05                                                                           1.32E-04                                     benzene                                                                             2.10E-04                                                                           1.09E-04                                                                            7.78E-05                                                                           1.07E-04                                                                            5.47E-05                                                                           2.40E-04                                     chloroform                                                                          2.68E-04                                                                           1.92E-04                                                                            1.08E-04                                                                           6.68E-05                                                                            1.54E-04                                                                           4.12E-04                                     cyclohexane                                                                         3.40E-04                                                                           8.72E-05                                                                            3.93E-05                                                                           6.75E-05                                                                            1.04E-04                                                                           1.27E-04                                     ethanol                                                                             1.04E-04                                                                           4.64E-06                                                                            4.02E-05                                                                           4.93E-05                                                                            6.15E-06                                                                           9.95E-05                                     hexane                                                                              7.78E-05                                                                           9.45E-05                                                                            9.02E-05                                                                           1.65E-04                                                                            4.74E-05                                                                           1.27E-04                                     methanol                                                                            4.35E-05                                                                           8.52E-06                                                                            3.66E-05                                                                           1.13E-04                                                                            2.92E-05                                                                           7.60E-05                                     toluene                                                                             7.89E-04                                                                           1.09E-04                                                                            1.97E-04                                                                           2.95E-04                                                                            7.18E-05                                                                           6.94E-04                                     AVERAGES                                                                      Sensor 8                                                                      poly(ethylene-vinyl acetate) (not heated)                                     2-propanol                                                                          1.98E-03                                                                           1.99E-03                                                                            2.09E-03                                                                           1.99E-03                                                                            2.15E-03                                                                           2.07E-03                                     benzene                                                                             6.30E-03                                                                           6.29E-03                                                                            6.64E-03                                                                           6.54E-03                                                                            6.65E-03                                                                           6.40E-03                                     chloroform                                                                          5.77E-03                                                                           5.68E-03                                                                            6.07E-03                                                                           6.05E-03                                                                            6.06E-03                                                                           5.89E-03                                     cyclohexane                                                                         5.09E-03                                                                           4.98E-03                                                                            5.33E-03                                                                           5.20E-03                                                                            5.29E-03                                                                           5.10E-03                                     ethanol                                                                             1.46E-03                                                                           1.48E-03                                                                            1.52E-03                                                                           1.51E-03                                                                            1.54E-03                                                                           1.51E-03                                     hexane                                                                              2.89E-03                                                                           2.71E-03                                                                            2.92E-03                                                                           3.01E-03                                                                            2.93E-03                                                                           2.76E-03                                     methanol                                                                            4.09E-04                                                                           3.81E-04                                                                            4.24E-04                                                                           4.08E-04                                                                            4.39E-04                                                                           4.05E-04                                     toluene                                                                             2.60E-02                                                                           2.59E-02                                                                            2.70E-02                                                                           2.63E-02                                                                            2.74E-02                                                                           2.59E-02                                     STANDARD DEVIATION                                                            Sensor 8                                                                      poly(ethylene-vinyl acetate) (not heated)                                     2-propanol                                                                          1.51E-04                                                                           2.74E-05                                                                            1.64E-05                                                                           1.01E-04                                                                            2.63E-05                                                                           1.54E-04                                     benzene                                                                             2.16E-04                                                                           8.58E-05                                                                            1.10E-04                                                                           1.35E-05                                                                            4.36E-05                                                                           3.07E-04                                     chloroform                                                                          3.16E-04                                                                           1.85E-04                                                                            1.32E-04                                                                           1.25E-04                                                                            2.21E-04                                                                           4.06E-04                                     cyclohexane                                                                         2.92E-04                                                                           5.74E-05                                                                            6.18E-05                                                                           6.35E-05                                                                            1.42E-04                                                                           1.59E-04                                     ethanol                                                                             1.06E-04                                                                           5.89E-06                                                                            2.59E-05                                                                           7.20E-05                                                                            1.26E-05                                                                           1.16E-04                                     hexane                                                                              1.22E-05                                                                           1.35E-04                                                                            9.43E-05                                                                           7.96E-05                                                                            6.86E-05                                                                           1.18E-04                                     methanol                                                                            5.87E-05                                                                           3.07E-05                                                                            4.54E-05                                                                           5.25E-05                                                                            4.22E-05                                                                           8.63E-05                                     toluene                                                                             8.10E-04                                                                           1.75E-04                                                                            2.71E-04                                                                           2.82E-04                                                                            1.08E-04                                                                           7.10E-04                                     __________________________________________________________________________    22° C.                                                                            22° C.                                                                       23° C.                                                                      45° C.                                                                       46° C.                                                                      55° C.                                __________________________________________________________________________    AVERAGES                                                                      Sensor 17                                                                     poly(ethylene-vinyl acetate)                                                  2-propanol                                                                          2.21E-03                                                                           2.41E-03                                                                            2.44E-03                                                                           2.98E-04                                                                            7.14E-04                                                                           2.53E-04                                     benzene                                                                             6.90E-03                                                                           7.49E-03                                                                            7.72E-03                                                                           1.21E-03                                                                            1.73E-03                                                                           7.61E-04                                     chloroform                                                                          6.49E-03                                                                           6.83E-03                                                                            7.07E-03                                                                           1.59E-03                                                                            1.77E-03                                                                           6.25E-04                                     cyclohexane                                                                         5.80E-03                                                                           6.20E-03                                                                            6.41E-03                                                                           1.97E-03                                                                            2.32E-03                                                                           9.23E-04                                     ethanol                                                                             1.54E-03                                                                           1.74E-03                                                                            1.72E-03                                                                           3.31E-04                                                                            5.19E-04                                                                           1.75E-04                                     hexane                                                                              3.33E-03                                                                           3.34E-03                                                                            3.46E-03                                                                           1.29E-03                                                                            1.18E-03                                                                           7.20E-04                                     methanol                                                                            4.19E-04                                                                           4.45E-04                                                                            4.55E-04                                                                           3.68E-05                                                                            5.85E-04                                                                           8.80E-05                                     toluene                                                                             2.72E-02                                                                           2.93E-02                                                                            2.98E-02                                                                           7.81E-03                                                                            6.29E-03                                                                           3.63E-03                                     STANDARD DEVIATION                                                            Sensor 17                                                                     poly(ethylene-vinyl acetate)                                                  2-propanol                                                                          1.90E-04                                                                           6.46E-05                                                                            2.05E-05                                                                           4.27E-04                                                                            2.19E-04                                                                           1.36E-04                                     benzene                                                                             3.13E-04                                                                           1.13E-04                                                                            1.38E-04                                                                           1.30E-03                                                                            4.55E-04                                                                           1.51E-04                                     chloroform                                                                          3.89E-04                                                                           2.87E-04                                                                            1.57E-04                                                                           1.75E-04                                                                            1.92E-04                                                                           1.77E-04                                     cyclohexane                                                                         4.04E-04                                                                           1.79E-04                                                                            8.12E-05                                                                           3.61E-06                                                                            3.20E-04                                                                           1.51E-04                                     ethanol                                                                             1.32E-04                                                                           4.09E-05                                                                            3.11E-05                                                                           1.22E-04                                                                            1.08E-04                                                                           1.11E-04                                     hexane                                                                              8.52E-05                                                                           1.86E-04                                                                            9.90E-05                                                                           2.44E-04                                                                            1.37E-05                                                                           3.93E-04                                     methanol                                                                            5.97E-05                                                                           3.50E-05                                                                            5.45E-05                                                                           9.24E-05                                                                            8.97E-04                                                                           1.46E-04                                     toluene                                                                             9.38E-04                                                                           3.33E-04                                                                            3.38E-04                                                                           3.07E-03                                                                            2.14E-04                                                                           2.95E-03                                     __________________________________________________________________________    22° C.                                                                            22° C.                                                                       23° C.                                                                      33° C.                                                                       34° C.                                                                      37° C.                                __________________________________________________________________________    AVERAGES                                                                      Sensor 20                                                                     poly(ethylene-vinyl acetate)                                                  2-propanol                                                                          3.83E-03                                                                           3.75E-03                                                                            3.91E-03                                                                           2.56E-03                                                                            2.89E-03                                                                           2.30E-03                                     benzene                                                                             1.09E-02                                                                           1.09E-02                                                                            1.12E-02                                                                           8.36E-03                                                                            8.20E-03                                                                           6.82E-03                                     chloroform                                                                          1.06E-02                                                                           1.02E-02                                                                            1.07E-02                                                                           7.63E-03                                                                            7.61E-03                                                                           5.87E-03                                     cyclohexane                                                                         9.50E-03                                                                           9.22E-03                                                                            9.63E-03                                                                           7.39E-03                                                                            7.52E-03                                                                           6.08E-03                                     ethanol                                                                             2.75E-03                                                                           2.82E-03                                                                            2.77E-03                                                                           2.00E-03                                                                            2.43E-03                                                                           1.82E-03                                     hexane                                                                              5.39E-03                                                                           4.93E-03                                                                            5.20E-03                                                                           4.20E-03                                                                            4.34E-03                                                                           3.13E-03                                     methanol                                                                            8.33E-04                                                                           7.91E-04                                                                            8.08E-04                                                                           6.09E-04                                                                            9.35E-04                                                                           5.63E-04                                     toluene                                                                             4.49E-02                                                                           4.37E-02                                                                            4.56E-02                                                                           3.01E-02                                                                            3.14E-02                                                                           2.35E-02                                     STANDARD DEVIATION                                                            Sensor 20                                                                     poly(ethylene-vinyl acetate)                                                  2-propanol                                                                          2.89E-04                                                                           6.98E-05                                                                            3.96E-05                                                                           2.04E-04                                                                            1.50E-04                                                                           5.24E-04                                     benzene                                                                             4.76E-04                                                                           1.63E-04                                                                            2.44E-04                                                                           6.45E-04                                                                            4.10E-04                                                                           5.20E-04                                     chloroform                                                                          6.01E-04                                                                           3.53E-04                                                                            2.61E-04                                                                           2.21E-04                                                                            3.29E-04                                                                           4.52E-04                                     cyclohexane                                                                         6.43E-06                                                                           1.88E-04                                                                            1.46E-04                                                                           3.20E-04                                                                            8.44E-05                                                                           6.69E-04                                     ethanol                                                                             2.17E-04                                                                           1.31E-05                                                                            5.29E-05                                                                           9.05E-05                                                                            3.28E-04                                                                           3.89E-04                                     hexane                                                                              1.71E-04                                                                           2.28E-04                                                                            1.38E-04                                                                           1.31E-04                                                                            1.42E-04                                                                           3.19E-04                                     methanol                                                                            5.76E-05                                                                           5.00E-05                                                                            2.52E-05                                                                           9.05E-05                                                                            9.12E-04                                                                           3.11E-04                                     toluene                                                                             1.75E-03                                                                           3.75E-04                                                                            5.35E-04                                                                           1.71E-03                                                                            5.16E-04                                                                           9.34E-04                                     __________________________________________________________________________    22° C.                                                                            22° C.                                                                       23° C.                                                                      22° C.                                                                       22° C.                                                                      22° C.                                __________________________________________________________________________    AVERAGES                                                                      Sensor 4                                                                      poly(ethylene oxide) (not heated)                                             2-propanol                                                                          3.34E-03                                                                           3.16E-03                                                                            3.34E-03                                                                           3.33E-03                                                                            3.28E-03                                                                           3.16E-03                                     benzene                                                                             8.18E-03                                                                           7.34E-03                                                                            8.06E-03                                                                           8.25E-03                                                                            7.86E-03                                                                           7.26E-03                                     chloroform                                                                          7.83E-03                                                                           7.17E-03                                                                            7.73E-03                                                                           8.01E-03                                                                            7.56E-03                                                                           7.20E-03                                     cyclohexane                                                                         9.32E-04                                                                           8.13E-04                                                                            9.19E-04                                                                           9.16E-04                                                                            8.74E-04                                                                           8.17E-04                                     ethanol                                                                             3.12E-03                                                                           2.94E-03                                                                            3.12E-03                                                                           3.09E-03                                                                            3.05E-03                                                                           2.90E-03                                     hexane                                                                              7.00E-04                                                                           5.70E-04                                                                            6.80E-04                                                                           6.86E-04                                                                            6.47E-04                                                                           6.05E-04                                     methanol                                                                            1.48E-03                                                                           1.38E-03                                                                            1.48E-03                                                                           1.47E-03                                                                            1.44E-03                                                                           1.39E-03                                     toluene                                                                             2.96E-02                                                                           2.65E-02                                                                            2.88E-02                                                                           2.93E-02                                                                            2.83E-02                                                                           2.62E-02                                     STANDARD DEVIATION                                                            Sensor 4                                                                      poly(ethylene oxide) (not heated)                                             2-propanol                                                                          1.26E-04                                                                           6.19E-05                                                                            3.44E-05                                                                           1.13E-05                                                                            6.71E-06                                                                           1.12E-04                                     benzene                                                                             2.33E-04                                                                           2.30E-04                                                                            7.16E-05                                                                           1.07E-05                                                                            6.90E-05                                                                           1.74E-04                                     chloroform                                                                          3.43E-04                                                                           3.64E-04                                                                            1.47E-04                                                                           1.26E-04                                                                            2.29E-04                                                                           2.85E-04                                     cyclohexane                                                                         1.72E-05                                                                           5.23E-05                                                                            1.83E-07                                                                           8.32E-06                                                                            1.81E-05                                                                           1.76E-05                                     ethanol                                                                             1.40E-04                                                                           1.37E-05                                                                            2.17E-05                                                                           2.32E-05                                                                            2.82E-05                                                                           1.33E-04                                     hexane                                                                              1.37E-05                                                                           6.93E-05                                                                            3.62E-06                                                                           3.07E-06                                                                            1.41E-05                                                                           2.07E-05                                     methanol                                                                            2.24E-05                                                                           2.22E-05                                                                            2.51E-05                                                                           2.03E-05                                                                            1.21E-05                                                                           4.49E-05                                     toluene                                                                             4.39E-04                                                                           1.17E-04                                                                            1.75E-04                                                                           2.24E-04                                                                            8.57E-05                                                                           3.94E-04                                     AVERAGES                                                                      Sensor 6                                                                      poly(ethylene oxide) (not heated)                                             2-propanol                                                                          3.30E-03                                                                           3.08E-03                                                                            3.29E-03                                                                           3.27E-03                                                                            3.21E-03                                                                           3.11E-03                                     benzene                                                                             9.14E-03                                                                           7.90E-03                                                                            8.92E-03                                                                           9.15E-03                                                                            8.61E-03                                                                           8.01E-03                                     chloroform                                                                          8.33E-03                                                                           7.45E-03                                                                            8.19E-03                                                                           8.42E-03                                                                            7.90E-03                                                                           7.58E-03                                     cyclohexane                                                                         1.05E-03                                                                           8.60E-04                                                                            1.02E-03                                                                           1.01E-03                                                                            9.59E-04                                                                           9.11E-04                                     ethanol                                                                             2.97E-03                                                                           2.78E-03                                                                            2.97E-03                                                                           2.94E-03                                                                            2.90E-03                                                                           2.78E-03                                     hexane                                                                              8.32E-04                                                                           6.13E-04                                                                            7.86E-04                                                                           7.95E-04                                                                            7.27E-04                                                                           7.00E-04                                     methanol                                                                            1.41E-03                                                                           1.29E-03                                                                            1.41E-03                                                                           1.40E-03                                                                            1.36E-03                                                                           1.32E-03                                     toluene                                                                             3.26E-02                                                                           2.86E-02                                                                            3.16E-02                                                                           3.21E-02                                                                            3.08E-02                                                                           2.86E-02                                     STANDARD DEVIATION                                                            Sensor 6                                                                      poly(ethylene oxide) (not heated)                                             2-propanol                                                                          1.41E-04                                                                           9.26E-05                                                                            3.57E-05                                                                           2.04E-05                                                                            1.50E-05                                                                           1.28E-04                                     benzene                                                                             2.67E-04                                                                           4.58E-04                                                                            7.88E-05                                                                           1.53E-05                                                                            9.19E-05                                                                           2.07E-04                                     chloroform                                                                          3.71E-04                                                                           4.83E-04                                                                            1.55E-04                                                                           1.30E-04                                                                            2.75E-04                                                                           3.12E-04                                     cyclohexane                                                                         2.33E-05                                                                           9.08E-05                                                                            7.15E-06                                                                           1.49E-05                                                                            2.55E-05                                                                           2.30E-05                                     ethanol                                                                             1.33E-04                                                                           4.14E-05                                                                            2.35E-05                                                                           1.28E-05                                                                            1.62E-05                                                                           1.31E-04                                     hexane                                                                              5.79E-06                                                                           1.20E-04                                                                            8.29E-06                                                                           6.28E-06                                                                            1.99E-05                                                                           1.83E-05                                     methanol                                                                            2.66E-05                                                                           3.85E-05                                                                            2.54E-05                                                                           1.50E-05                                                                            1.27E-05                                                                           5.24E-05                                     toluene                                                                             4.69E-04                                                                           4.37E-04                                                                            1.57E-04                                                                           2.68E-04                                                                            1.32E-04                                                                           4.26E-04                                     AVERAGES                                                                      Sensor 9                                                                      poly(ethylene oxide) (not heated)                                             2-propanol                                                                          3.22E-03                                                                           3.50E-03                                                                            3.30E-03                                                                           3.27E-03                                                                            3.17E-03                                                                           3.44E-03                                     benzene                                                                             6.27E-03                                                                           5.77E-03                                                                            6.10E-03                                                                           6.32E-03                                                                            5.80E-03                                                                           5.19E-03                                     chloroform                                                                          6.60E-03                                                                           6.27E-03                                                                            6.51E-03                                                                           6.78E-03                                                                            6.37E-03                                                                           5.95E-03                                     cyclohexane                                                                         7.82E-04                                                                           9.06E-04                                                                            7.59E-04                                                                           7.55E-04                                                                            7.16E-04                                                                           6.71E-04                                     ethanol                                                                             3.03E-03                                                                           3.22E-03                                                                            2.98E-03                                                                           2.96E-03                                                                            2.99E-03                                                                           3.02E-03                                     hexane                                                                              5.46E-04                                                                           8.12E-04                                                                            5.37E-04                                                                           5.84E-04                                                                            5.11E-04                                                                           5.61E-04                                     methanol                                                                            1.50E-03                                                                           1.75E-03                                                                            1.45E-03                                                                           1.51E-03                                                                            1.55E-03                                                                           1.63E-03                                     toluene                                                                             2.20E-02                                                                           1.84E-02                                                                            2.11E-02                                                                           2.18E-02                                                                            1.99E-02                                                                           1.72E-02                                     STANDARD DEVIATION                                                            Sensor 9                                                                      poly(ethylene oxide) (not heated)                                             2-propanol                                                                          1.61E-04                                                                           1.51E-04                                                                            7.00E-05                                                                           2.98E-05                                                                            1.16E-04                                                                           3.78E-04                                     benzene                                                                             2.27E-04                                                                           4.88E-04                                                                            1.35E-04                                                                           1.08E-04                                                                            4.08E-05                                                                           9.07E-05                                     chloroform                                                                          2.06E-04                                                                           1.36E-04                                                                            5.63E-05                                                                           6.31E-05                                                                            1.40E-04                                                                           4.07E-04                                     cyclohexane                                                                         5.07E-05                                                                           1.66E-04                                                                            3.37E-05                                                                           9.69E-05                                                                            9.88E-06                                                                           3.78E-05                                     ethanol                                                                             1.73E-04                                                                           2.07E-04                                                                            1.65E-05                                                                           6.02E-05                                                                            1.96E-05                                                                           1.85E-04                                     hexane                                                                              8.37E-05                                                                           2.83E-04                                                                            4.77E-05                                                                           4.83E-05                                                                            1.68E-04                                                                           1.64E-04                                     methanol                                                                            7.56E-05                                                                           1.00E-04                                                                            2.88E-05                                                                           5.50E-05                                                                            8.35E-05                                                                           1.31E-04                                     toluene                                                                             4.26E-04                                                                           3.61E-04                                                                            8.15E-05                                                                           2.75E-04                                                                            4.39E-04                                                                           3.32E-04                                     __________________________________________________________________________    22° C.                                                                            22° C.                                                                       23° C.                                                                      44° C.                                                                       47° C.                                                                      57° C.                                __________________________________________________________________________    AVERAGES                                                                      Sensor 15                                                                     poly(ethylene oxide)                                                          2-propanol                                                                          2.77E-03                                                                           2.98E-03                                                                            2.72E-03                                                                           7.13E-04                                                                            9.59E-04                                                                           3.19E-04                                     benzene                                                                             2.84E-03                                                                           3.10E-03                                                                            2.68E-03                                                                           9.04E-04                                                                            1.20E-03                                                                           3.39E-04                                     chloroform                                                                          3.58E-03                                                                           3.69E-03                                                                            3.36E-03                                                                           1.08E-03                                                                            1.24E-03                                                                           3.68E-04                                     cyclohexane                                                                         5.61E-04                                                                           8.60E-04                                                                            5.85E-04                                                                           1.34E-04                                                                            3.74E-04                                                                           1.79E-04                                     ethanol                                                                             2.57E-03                                                                           2.79E-03                                                                            2.47E-03                                                                           7.74E-04                                                                            7.59E-04                                                                           3.65E-04                                     hexane                                                                              4.32E-04                                                                           6.88E-04                                                                            4.27E-04                                                                           2.39E-04                                                                            2.71E-04                                                                           7.47E-05                                     methanol                                                                            1.30E-03                                                                           1.56E-03                                                                            1.27E-03                                                                           4.09E-04                                                                            4.07E-04                                                                           2.53E-04                                     toluene                                                                             9.45E-03                                                                           8.67E-03                                                                            8.77E-03                                                                           3.66E-03                                                                            3.18E-03                                                                           1.29E-03                                     STANDARD DEVIATION                                                            Sensor 15                                                                     poly(ethylene oxide)                                                          2-propanol                                                                          7.33E-05                                                                           3.80E-05                                                                            3.75E-05                                                                           2.57E-04                                                                            6.46E-05                                                                           9.75E-05                                     benzene                                                                             2.31E-05                                                                           4.43E-04                                                                            5.95E-05                                                                           3.78E-04                                                                            1.52E-04                                                                           2.56E-04                                     chloroform                                                                          1.11E-04                                                                           1.88E-04                                                                            2.75E-05                                                                           1.65E-04                                                                            9.19E-05                                                                           1.11E-04                                     cyclohexane                                                                         2.69E-05                                                                           1.20E-04                                                                            1.32E-04                                                                           4.88E-05                                                                            1.96E-04                                                                           2.41E-04                                     ethanol                                                                             8.53E-05                                                                           1.61E-04                                                                            1.35E-04                                                                           8.05E-05                                                                            9.10E-05                                                                           2.14E-04                                     hexane                                                                              8.14E-05                                                                           9.59E-05                                                                            1.24E-05                                                                           2.91E-04                                                                            1.21E-04                                                                           3.69E-05                                     methanol                                                                            7.68E-05                                                                           3.61E-05                                                                            1.24E-04                                                                           7.45E-05                                                                            1.30E-04                                                                           1.33E-04                                     toluene                                                                             1.23E-04                                                                           3.63E-04                                                                            9.63E-05                                                                           7.71E-05                                                                            1.43E-04                                                                           2.46E-04                                     __________________________________________________________________________    22° C.                                                                            22° C.                                                                       23° C.                                                                      33° C.                                                                       33° C.                                                                      37° C.                                __________________________________________________________________________    AVERAGES                                                                      Sensor 16                                                                     poly(ethylene oxide)                                                          2-propanol                                                                          3.02E-03                                                                           3.03E-03                                                                            2.96E-03                                                                           1.84E-03                                                                            1.81E-03                                                                           1.35E-03                                     benzene                                                                             6.36E-03                                                                           5.48E-03                                                                            5.99E-03                                                                           4.56E-03                                                                            4.14E-03                                                                           2.95E-03                                     chloroform                                                                          6.49E-03                                                                           5.86E-03                                                                            6.18E-03                                                                           4.20E-03                                                                            3.87E-03                                                                           2.77E-03                                     cyclohexane                                                                         6.70E-04                                                                           6.68E-04                                                                            6.34E-04                                                                           5.19E-04                                                                            4.89E-04                                                                           4.13E-04                                     ethanol                                                                             2.74E-03                                                                           2.81E-03                                                                            2.68E-03                                                                           1.71E-03                                                                            1.66E-03                                                                           1.28E-03                                     hexane                                                                              4.22E-04                                                                           5.13E-04                                                                            4.37E-04                                                                           3.48E-04                                                                            3.53E-04                                                                           3.02E-04                                     methanol                                                                            1.32E-03                                                                           1.42E-03                                                                            1.27E-03                                                                           8.77E-04                                                                            8.50E-04                                                                           7.23E-04                                     toluene                                                                             2.28E-02                                                                           1.87E-02                                                                            2.08E-02                                                                           1.61E-02                                                                            1.43E-02                                                                           1.03E-02                                     STANDARD DEVIATION                                                            Sensor 16                                                                     poly(ethylene oxide)                                                          2-propanol                                                                          1.18E-04                                                                           7.87E-05                                                                            2.67E-05                                                                           4.18E-05                                                                            2.22E-05                                                                           4.92E-05                                     benzene                                                                             2.12E-04                                                                           1.65E-04                                                                            4.59E-05                                                                           3.95E-05                                                                            6.30E-05                                                                           1.12E-04                                     chloroform                                                                          3.17E-04                                                                           8.02E-05                                                                            1.20E-04                                                                           4.67E-05                                                                            6.31E-05                                                                           1.40E-04                                     cyclohexane                                                                         7.01E-06                                                                           7.16E-05                                                                            8.73E-06                                                                           4.19E-05                                                                            4.15E-05                                                                           1.80E-05                                     ethanol                                                                             1.27E-04                                                                           1.07E-04                                                                            1.53E-05                                                                           2.90E-05                                                                            1.12E-05                                                                           4.05E-05                                     hexane                                                                              2.13E-05                                                                           7.84E-05                                                                            4.94E-06                                                                           2.98E-05                                                                            2.28E-05                                                                           3.25E-05                                     methanol                                                                            2.01E-05                                                                           5.03E-05                                                                            3.18E-05                                                                           1.20E-05                                                                            4.36E-05                                                                           1.71E-05                                     toluene                                                                             4.12E-04                                                                           2.44E-04                                                                            1.78E-04                                                                           2.16E-04                                                                            1.77E-04                                                                           1.18E-04                                     __________________________________________________________________________    22° C.                                                                            22° C.                                                                       23° C.                                                                      22° C.                                                                       22° C.                                                                      22° C.                                __________________________________________________________________________    AVERAGES                                                                      Sensor 5                                                                      poly(4-vinyl phenol) (not heated)                                             2-propanol                                                                          3.31E-03                                                                           1.60E-03                                                                            2.56E-03                                                                           3.18E-03                                                                            2.44E-03                                                                           1.53E-03                                     benzene                                                                             5.11E-04                                                                           3.45E-04                                                                            4.77E-04                                                                           5.23E-04                                                                            4.47E-04                                                                           3.93E-04                                     chloroform                                                                          9.93E-04                                                                           6.38E-04                                                                            9.01E-04                                                                           1.05E-03                                                                            8.95E-04                                                                           5.96E-04                                     cyclohexane                                                                         1.32E-04                                                                           7.83E-05                                                                            9.96E-05                                                                           7.61E-05                                                                            8.85E-05                                                                           1.01E-04                                     ethanol                                                                             1.75E-02                                                                           1.33E-02                                                                            1.57E-02                                                                           1.67E-02                                                                            1.51E-02                                                                           1.27E-02                                     hexane                                                                              1.33E-04                                                                           1.05E-04                                                                            1.20E-04                                                                           1.74E-04                                                                            1.10E-04                                                                           1.07E-04                                     methanol                                                                            1.27E-02                                                                           1.15E-02                                                                            1.18E-02                                                                           1.23E-02                                                                            1.18E-02                                                                           1.11E-02                                     toluene                                                                             1.08E-03                                                                           7.02E-04                                                                            9.74E-04                                                                           1.08E-03                                                                            8.69E-04                                                                           6.69E-04                                     STANDARD DEVIATION                                                            Sensor 5                                                                      poly(4-vinyl phenol) (not heated)                                             2-propanol                                                                          1.26E-04                                                                           7.17E-05                                                                            5.29E-05                                                                           7.47E-05                                                                            8.36E-05                                                                           1.09E-04                                     benzene                                                                             1.84E-05                                                                           2.92E-05                                                                            4.24E-05                                                                           4.00E-05                                                                            1.96E-05                                                                           2.42E-05                                     chloroform                                                                          6.73E-05                                                                           3.52E-05                                                                            2.11E-05                                                                           5.81E-05                                                                            2.35E-05                                                                           4.43E-05                                     cyclohexane                                                                         1.52E-05                                                                           3.30E-05                                                                            1.33E-05                                                                           5.11E-05                                                                            1.66E-05                                                                           6.08E-05                                     ethanol                                                                             5.63E-04                                                                           5.20E-04                                                                            1.40E-04                                                                           5.19E-04                                                                            3.34E-04                                                                           6.10E-04                                     hexane                                                                              3.54E-05                                                                           9.39E-06                                                                            2.96E-05                                                                           3.18E-05                                                                            3.84E-06                                                                           2.68E-05                                     methanol                                                                            5.31E-05                                                                           1.78E-04                                                                            1.50E-04                                                                           1.62E-04                                                                            9.92E-05                                                                           3.93E-04                                     toluene                                                                             5.06E-05                                                                           5.00E-05                                                                            9.14E-05                                                                           9.22E-05                                                                            4.81E-05                                                                           5.81E-06                                     AVERAGES                                                                      Sensor 7                                                                      poly(4-vinyl phenol) (not heated)                                             2-propanol                                                                          1.55E-03                                                                           2.49E-04                                                                            1.10E-03                                                                           1.47E-03                                                                            7.27E-04                                                                           2.63E-04                                     benzene                                                                             6.95E-04                                                                           2.83E-04                                                                            4.63E-04                                                                           4.46E-04                                                                            3.87E-04                                                                           3.08E-04                                     chloroform                                                                          5.86E-04                                                                           2.19E-04                                                                            4.86E-04                                                                           6.59E-04                                                                            3.87E-04                                                                           2.11E-04                                     cyclohexane                                                                         3.35E-04                                                                           8.02E-05                                                                            1.17E-04                                                                           1.61E-04                                                                            8.90E-05                                                                           1.43E-04                                     ethanol                                                                             1.20E-02                                                                           4.88E-03                                                                            1.06E-02                                                                           1.17E-02                                                                            8.79E-03                                                                           4.38E-03                                     hexane                                                                              1.84E-04                                                                           1.60E-04                                                                            2.16E-04                                                                           1.72E-04                                                                            1.57E-04                                                                           1.34E-04                                     methanol                                                                            1.09E-02                                                                           1.02E-02                                                                            1.10E-02                                                                           1.13E-02                                                                            1.29E-02                                                                           1.04E-02                                     toluene                                                                             1.06E-03                                                                           6.43E-04                                                                            9.00E-04                                                                           9.33E-04                                                                            7.33E-04                                                                           5.74E-04                                     STANDARD DEVIATION                                                            Sensor 7                                                                      poly(4-vinyl phenol) (not heated)                                             2-propanol                                                                          1.36E-04                                                                           4.15E-05                                                                            8.21E-05                                                                           1.56E-04                                                                            5.48E-05                                                                           5.45E-05                                     benzene                                                                             1.16E-05                                                                           8.23E-05                                                                            8.53E-05                                                                           1.03E-04                                                                            8.77E-05                                                                           4.20E-05                                     chloroform                                                                          6.41E-05                                                                           1.21E-04                                                                            7.59E-05                                                                           1.31E-04                                                                            1.36E-04                                                                           3.63E-05                                     cyclohexane                                                                         4.14E-05                                                                           3.62E-05                                                                            5.71E-05                                                                           6.57E-05                                                                            3.68E-05                                                                           8.23E-05                                     ethanol                                                                             8.41E-04                                                                           4.75E-04                                                                            2.43E-04                                                                           4.28E-04                                                                            5.34E-04                                                                           2.41E-04                                     hexane                                                                              1.12E-04                                                                           5.94E-05                                                                            9.81E-05                                                                           2.63E-05                                                                            4.34E-05                                                                           6.98E-05                                     methanol                                                                            1.97E-04                                                                           5.53E-04                                                                            2.48E-04                                                                           3.93E-04                                                                            5.12E-04                                                                           2.80E-04                                     toluene                                                                             8.16E-05                                                                           8.98E-05                                                                            9.25E-05                                                                           3.72E-05                                                                            8.43E-05                                                                           7.97E-05                                     __________________________________________________________________________    22° C.                                                                            22° C.                                                                       23° C.                                                                      45° C.                                                                       46° C.                                                                      64° C.                                __________________________________________________________________________    AVERAGES                                                                      Sensor 18                                                                     poly(4-vinyl phenol)                                                          2-propanol                                                                          7.57E-04                                                                           1.57E-03                                                                            9.23E-04                                                                           2.45E-03                                                                            1.42E-03                                                                           4.89E-03                                     benzene                                                                             3.58E-04                                                                           2.91E-03                                                                            2.76E-03                                                                           1.12E-03                                                                            3.35E-03                                                                           2.62E-03                                     chloroform                                                                          3.12E-03                                                                           6.87E-03                                                                            2.50E-03                                                                           3.91E-03                                                                            1.73E-03                                                                           4.88E-03                                     cyclohexane                                                                         2.72E-03                                                                           8.70E-03                                                                            1.27E-03                                                                           1.67E-03                                                                            2.42E-03                                                                           3.45E-03                                     ethanol                                                                             4.83E-03                                                                           4.53E-03                                                                            5.14E-03                                                                           1.50E-02                                                                            1.81E-03                                                                           1.03E-02                                     hexane                                                                              1.81E-03                                                                           3.69E-03                                                                            1.27E-03                                                                           3.42E-03                                                                            2.12E-03                                                                           5.06E-03                                     methanol                                                                            3.18E-02                                                                           1.77E-02                                                                            3.93E-02                                                                           2.54E-02                                                                            3.42E-02                                                                           1.20E-02                                     toluene                                                                             3.89E-03                                                                           2.17E-03                                                                            1.16E-03                                                                           -9.19E-04                                                                           3.59E-03                                                                           2.81E-03                                     STANDARD DEVIATION                                                            Sensor 18                                                                     poly(4-vinyl phenol)                                                          2-propanol                                                                          4.19E-04                                                                           2.50E-03                                                                            4.77E-04                                                                           9.88E-04                                                                            2.29E-03                                                                           3.09E-03                                     benzene                                                                             3.18E-03                                                                           3.65E-03                                                                            1.91E-03                                                                           2.53E-03                                                                            1.78E-04                                                                           1.98E-03                                     chloroform                                                                          2.20E-03                                                                           3.96E-04                                                                            1.63E-03                                                                           3.21E-03                                                                            1.08E-03                                                                           7.30E-04                                     cyclohexane                                                                         2.31E-03                                                                           1.17E-02                                                                            1.56E-03                                                                           4.69E-03                                                                            8.68E-04                                                                           1.71E-03                                     ethanol                                                                             1.61E-03                                                                           2.37E-03                                                                            9.72E-04                                                                           2.33E-03                                                                            2.40E-03                                                                           9.56E-03                                     hexane                                                                              1.01E-03                                                                           2.80E-03                                                                            6.31E-04                                                                           1.70E-03                                                                            8.44E-04                                                                           1.95E-03                                     methanol                                                                            8.72E-03                                                                           5.25E-03                                                                            1.23E-03                                                                           1.11E-02                                                                            4.57E-03                                                                           5.97E-03                                     toluene                                                                             3.90E-03                                                                           5.32E-03                                                                            9.34E-04                                                                           3.75E-03                                                                            1.47E-03                                                                           2.99E-04                                     __________________________________________________________________________    22° C.                                                                            22° C.                                                                       23° C.                                                                      37° C.                                                                       38° C.                                                                      43° C.                                __________________________________________________________________________    AVERAGES                                                                      Sensor 19                                                                     poly(4-vinyl phenol)                                                          2-propanol                                                                          9.09E-04                                                                           2.38E-04                                                                            1.86E-04                                                                           1.02E-03                                                                            2.74E-04                                                                           1.72E-04                                     benzene                                                                             2.32E-04                                                                           2.14E-04                                                                            2.22E-04                                                                           2.58E-04                                                                            1.52E-04                                                                           1.28E-04                                     chloroform                                                                          3.91E-04                                                                           1.31E-04                                                                            1.69E-04                                                                           3.29E-04                                                                            1.76E-04                                                                           8.66E-05                                     cyclohexane                                                                         1.31E-04                                                                           8.34E-05                                                                            1.16E-04                                                                           4.71E-05                                                                            7.94E-05                                                                           1.25E-04                                     ethanol                                                                             1.09E-02                                                                           6.48E-03                                                                            5.48E-03                                                                           7.68E-03                                                                            4.81E-03                                                                           2.38E-03                                     hexane                                                                              8.28E-05                                                                           9.80E-05                                                                            1.03E-04                                                                           4.48E-05                                                                            7.25E-05                                                                           1.66E-04                                     methanol                                                                            1.06E-02                                                                           9.98E-03                                                                            9.23E-03                                                                           6.73E-03                                                                            5.82E-03                                                                           4.04E-03                                     toluene                                                                             4.57E-04                                                                           4.10E-04                                                                            4.62E-04                                                                           3.51E-04                                                                            2.17E-04                                                                           1.62E-04                                     STANDARD DEVIATION                                                            Sensor 19                                                                     poly(4-vinyl phenol)                                                          2-propanol                                                                          1.62E-05                                                                           4.38E-05                                                                            6.09E-05                                                                           5.69E-05                                                                            1.25E-04                                                                           8.61E-05                                     benzene                                                                             1.60E-05                                                                           7.93E-05                                                                            6.85E-05                                                                           1.43E-04                                                                            4.41E-05                                                                           2.55E-05                                     chloroform                                                                          6.82E-05                                                                           2.86E-05                                                                            1.82E-05                                                                           2.95E-05                                                                            9.03E-05                                                                           3.69E-05                                     cyclohexane                                                                         1.32E-05                                                                           4.94E-05                                                                            7.90E-05                                                                           6.32E-05                                                                            3.30E-05                                                                           3.08E-05                                     ethanol                                                                             5.51E-04                                                                           5.99E-04                                                                            3.17E-05                                                                           7.08E-04                                                                            5.44E-04                                                                           4.33E-04                                     hexane                                                                              4.84E-05                                                                           6.92E-06                                                                            3.30E-05                                                                           9.91E-05                                                                            1.03E-05                                                                           7.23E-05                                     methanol                                                                            2.21E-04                                                                           4.53E-04                                                                            1.04E-04                                                                           5.95E-04                                                                            2.21E-04                                                                           3.94E-04                                     toluene                                                                             9.02E-05                                                                           2.26E-05                                                                            1.02E-04                                                                           1.68E-05                                                                            1.38E-04                                                                           1.72E-04                                     __________________________________________________________________________

                                      TABLE 7                                     __________________________________________________________________________    Exp 1       Exp 3                                                                              Exp 5 Exp 2                                                                              Exp 4 Exp 6                                       __________________________________________________________________________    22° C.                                                                             22° C.                                                                      23° C.                                                                       22° C.                                                                      22° C.                                                                       22° C.                               __________________________________________________________________________    Normalized averages                                                           Sensor 3                                                                      poly(ethylene-vinyl acetate) (not heated)                                     2-propanol                                                                          3.67E-02                                                                            3.69E-02                                                                           3.73E-02                                                                            3.63E-02                                                                           3.80E-02                                                                            3.82E-02                                    benzene                                                                             1.27E-01                                                                            1.28E-01                                                                           1.27E-01                                                                            1.29E-01                                                                           1.27E-01                                                                            1.29E-01                                    chloroform                                                                          1.15E-01                                                                            1.13E-01                                                                           1.16E-01                                                                            1.18E-01                                                                           1.14E-01                                                                            1.16E-01                                    cyclohexane                                                                         1.03E-01                                                                            1.03E-01                                                                           1.04E-01                                                                            1.02E-01                                                                           1.03E-01                                                                            1.05E-01                                    ethanol                                                                             2.70E-02                                                                            2.76E-02                                                                           2.73E-02                                                                            2.77E-02                                                                           2.73E-02                                                                            2.77E-02                                    hexane                                                                              5.97E-02                                                                            5.63E-02                                                                           5.67E-02                                                                            5.99E-02                                                                           5.75E-02                                                                            5.72E-02                                    methanol                                                                            6.71E-03                                                                            6.76E-03                                                                           6.89E-03                                                                            8.42E-03                                                                           7.37E-03                                                                            6.74E-03                                    toluene                                                                             5.25E-01                                                                            5.28E-01                                                                           5.25E-01                                                                            5.19E-01                                                                           5.25E-01                                                                            5.21E-01                                    Normalized STANDARD DEVIATION                                                 Sensor 3                                                                      poly(ethylene-vinyl acetate) (not heated)                                     2-propanol                                                                          2.66E-03                                                                            7.46E-04                                                                           3.37E-04                                                                            2.26E-03                                                                           7.96E-04                                                                            2.75E-03                                    benzene                                                                             4.25E-03                                                                            2.31E-03                                                                           1.52E-03                                                                            2.12E-03                                                                           1.07E-03                                                                            4.98E-03                                    chloroform                                                                          5.44E-03                                                                            4.04E-03                                                                           2.12E-03                                                                            1.32E-03                                                                           3.03E-03                                                                            8.57E-03                                    cyclohexane                                                                         6.89E-03                                                                            1.84E-03                                                                           7.68E-04                                                                            1.33E-03                                                                           2.04E-03                                                                            2.64E-03                                    ethanol                                                                             2.11E-03                                                                            9.77E-05                                                                           7.85E-04                                                                            9.72E-04                                                                           1.21E-04                                                                            2.07E-03                                    hexane                                                                              1.58E-03                                                                            1.99E-03                                                                           1.76E-03                                                                            3.26E-03                                                                           9.29E-04                                                                            2.63E-03                                    methanol                                                                            8.81E-04                                                                            1.80E-04                                                                           7.15E-04                                                                            2.22E-03                                                                           5.73E-04                                                                            1.58E-03                                    toluene                                                                             1.60E-02                                                                            2.29E-03                                                                           3.84E-03                                                                            5.82E-03                                                                           1.41E-03                                                                            1.44E-02                                    Normalized averages                                                           Sensor 8                                                                      poly(ethylene-vinyl acetate) (not heated)                                     2-propanol                                                                          3.97E-02                                                                            4.02E-02                                                                           4.01E-02                                                                            3.91E-02                                                                           4.10E-02                                                                            4.13E-02                                    benzene                                                                             1.26E-01                                                                            1.27E-01                                                                           1.28E-01                                                                            1.28E-01                                                                           1.27E-01                                                                            1.28E-01                                    chloroform                                                                          1.16E-01                                                                            1.15E-01                                                                           1.17E-01                                                                            1.19E-01                                                                           1.16E-01                                                                            1.18E-01                                    cyclohexane                                                                         1.02E-01                                                                            1.01E-01                                                                           1.02E-01                                                                            1.02E-01                                                                           1.01E-01                                                                            1.02E-01                                    ethanol                                                                             2.93E-02                                                                            3.00E-02                                                                           2.91E-02                                                                            2.96E-02                                                                           2.94E-02                                                                            3.01E-02                                    hexane                                                                              5.79E-02                                                                            5.48E-02                                                                           5.61E-02                                                                            5.90E-02                                                                           5.60E-02                                                                            5.53E-02                                    methanol                                                                            8.20E-03                                                                            7.71E-03                                                                           8.14E-03                                                                            8.01E-03                                                                           8.37E-03                                                                            8.10E-03                                    toluene                                                                             5.21E-01                                                                            5.25E-01                                                                           5.20E-01                                                                            5.16E-01                                                                           5.22E-01                                                                            5.17E-01                                    Normalized STANDARD DEVIATION                                                 Sensor 8                                                                      poly(ethylene-vinyl acetate) (not heated)                                     2-propanol                                                                          3.03E-03                                                                            5.54E-04                                                                           3.15E-04                                                                            1.98E-03                                                                           5.01E-04                                                                            3.08E-03                                    benzene                                                                             4.33E-03                                                                            1.74E-03                                                                           2.12E-03                                                                            2.65E-04                                                                           8.33E-04                                                                            6.15E-03                                    chloroform                                                                          6.34E-03                                                                            3.74E-03                                                                           2.54E-03                                                                            2.46E-03                                                                           4.21E-03                                                                            8.12E-03                                    cyclohexane                                                                         5.86E-03                                                                            1.16E-03                                                                           1.19E-03                                                                            1.25E-03                                                                           2.70E-03                                                                            3.18E-03                                    ethanol                                                                             2.13E-03                                                                            1.19E-04                                                                           4.98E-04                                                                            1.41E-03                                                                           2.40E-04                                                                            2.32E-03                                    hexane                                                                              2.45E-04                                                                            2.73E-03                                                                           1.81E-03                                                                            1.56E-03                                                                           1.31E-03                                                                            2.36E-03                                    methanol                                                                            1.18E-03                                                                            6.21E-04                                                                           8.73E-04                                                                            1.03E-03                                                                           8.06E-04                                                                            1.73E-03                                    toluene                                                                             1.62E-02                                                                            3.54E-03                                                                           5.20E-03                                                                            5.53E-03                                                                           2.06E-03                                                                            1.42E-02                                    __________________________________________________________________________    22° C.                                                                             22° C.                                                                      23° C.                                                                       45° C.                                                                      46° C.                                                                       55° C.                               __________________________________________________________________________    Normalized averages                                                           Sensor 17                                                                     poly(ethylene-vinyl acetate)                                                  2-propanol                                                                          4.10E-02                                                                            4.17E-02                                                                           4.13E-02                                                                            2.05E-02                                                                           4.72E-02                                                                            3.52E-02                                    benzene                                                                             1.28E-01                                                                            1.30E-01                                                                           1.31E-01                                                                            8.31E-02                                                                           1.15E-01                                                                            1.06E-01                                    chloroform                                                                          1.20E-01                                                                            1.18E-01                                                                           1.20E-01                                                                            1.09E-01                                                                           1.17E-01                                                                            8.71E-02                                    cyclohexane                                                                         1.08E-01                                                                            1.07E-01                                                                           1.08E-01                                                                            1.35E-01                                                                           1.54E-01                                                                            1.29E-01                                    ethanol                                                                             2.86E-02                                                                            3.02E-02                                                                           2.91E-02                                                                            2.28E-02                                                                           3.43E-02                                                                            2.43E-02                                    hexane                                                                              6.18E-02                                                                            5.79E-02                                                                           5.85E-02                                                                            8.87E-02                                                                           7.79E-02                                                                            1.00E-01                                    methanol                                                                            7.78E-03                                                                            7.71E-03                                                                           7.70E-03                                                                            2.53E-03                                                                           3.87E-02                                                                            1.23E-02                                    toluene                                                                             5.04E-01                                                                            5.07E-01                                                                           5.05E-01                                                                            5.37E-01                                                                           4.16E-01                                                                            5.06E-01                                    Normalized STANDARD DEVIATION                                                 Sensor 17                                                                     poly(ethylene-vinyl acetate)                                                  2-propanol                                                                          3.52E-03                                                                            1.12E-03                                                                           3.46E-04                                                                            2.94E-02                                                                           1.45E-02                                                                            1.90E-02                                    benzene                                                                             5.81E-03                                                                            1.97E-03                                                                           2.33E-03                                                                            8.94E-02                                                                           3.01E-02                                                                            2.11E-02                                    chloroform                                                                          7.22E-03                                                                            4.96E-03                                                                           2.66E-03                                                                            1.20E-02                                                                           1.27E-02                                                                            2.47E-02                                    cyclohexane                                                                         7.49E-03                                                                            3.11E-03                                                                           1.37E-03                                                                            2.48E-04                                                                           2.12E-02                                                                            2.10E-02                                    ethanol                                                                             2.44E-03                                                                            7.09E-04                                                                           5.26E-04                                                                            8.40E-03                                                                           7.16E-03                                                                            1.55E-02                                    hexane                                                                              1.58E-03                                                                            3.23E-03                                                                           1.67E-03                                                                            1.68E-02                                                                           9.06E-04                                                                            5.47E-02                                    methanol                                                                            1.11E-03                                                                            6.06E-04                                                                           9.22E-04                                                                            6.36E-03                                                                           5.94E-02                                                                            2.04E-02                                    toluene                                                                             1.74E-02                                                                            5.77E-03                                                                           5.71E-03                                                                            2.11E-01                                                                           1.42E-02                                                                            4.10E-01                                    __________________________________________________________________________    22° C.                                                                             22° C.                                                                      23° C.                                                                       33° C.                                                                      34° C.                                                                       37° C.                               __________________________________________________________________________    Normalized averages                                                           Sensor 20                                                                     poly(ethylene-vinyl acetate)                                                  2-propanol                                                                          4.31E-02                                                                            4.35E-02                                                                           4.35E-02                                                                            4.08E-02                                                                           4.42E-02                                                                            4.60E-02                                    benzene                                                                             1.23E-01                                                                            1.26E-01                                                                           1.25E-01                                                                            1.33E-01                                                                           1.25E-01                                                                            1.36E-01                                    chloroform                                                                          1.20E-01                                                                            1.18E-01                                                                           1.19E-01                                                                            1.21E-01                                                                           1.16E-01                                                                            1.17E-01                                    cyclohexane                                                                         1.07E-01                                                                            1.07E-01                                                                           1.07E-01                                                                            1.18E-01                                                                           1.15E-01                                                                            1.21E-01                                    ethanol                                                                             3.10E-02                                                                            3.27E-02                                                                           3.08E-02                                                                            3.18E-02                                                                           3.73E-02                                                                            3.63E-02                                    hexane                                                                              6.07E-02                                                                            5.72E-02                                                                           5.79E-02                                                                            6.68E-02                                                                           6.64E-02                                                                            6.26E-02                                    methanol                                                                            9.39E-03                                                                            9.17E-03                                                                           8.99E-03                                                                            9.69E-03                                                                           1.43E-02                                                                            1.12E-02                                    toluene                                                                             5.06E-01                                                                            5.06E-01                                                                           5.07E-01                                                                            4.79E-01                                                                           4.81E-01                                                                            4.69E-01                                    Normalized STANDARD DEVIATION                                                 Sensor 20                                                                     poly(ethylene-vinyl acetate)                                                  2-propanol                                                                          3.26E-03                                                                            8.09E-04                                                                           4.41E-04                                                                            3.24E-03                                                                           2.30E-03                                                                            1.05E-02                                    benzene                                                                             5.36E-03                                                                            1.89E-03                                                                           2.72E-03                                                                            1.03E-02                                                                           6.27E-03                                                                            1.04E-02                                    chloroform                                                                          6.77E-03                                                                            4.09E-03                                                                           2.90E-03                                                                            3.51E-03                                                                           5.03E-03                                                                            9.04E-03                                    cyclohexane                                                                         7.25E-03                                                                            2.18E-03                                                                           1.63E-03                                                                            5.10E-03                                                                           1.29E-03                                                                            1.34E-02                                    ethanol                                                                             2.45E-03                                                                            1.51E-04                                                                           5.88E-04                                                                            1.44E-03                                                                           5.03E-03                                                                            7.77E-03                                    hexane                                                                              1.93E-03                                                                            2.64E-03                                                                           1.54E-03                                                                            2.09E-03                                                                           2.17E-03                                                                            6.38E-03                                    methanol                                                                            6.49E-04                                                                            5.80E-04                                                                           2.81E-04                                                                            1.44E-03                                                                           1.40E-02                                                                            6.22E-03                                    toluene                                                                             1.98E-02                                                                            4.35E-03                                                                           5.95E-03                                                                            2.72E-02                                                                           7.90E-03                                                                            1.86E-02                                    __________________________________________________________________________    22° C.                                                                             22° C.                                                                      23° C.                                                                       22° C.                                                                      22° C.                                                                       22° C.                               __________________________________________________________________________    Normalized averages                                                           Sensor 4                                                                      poly(ethylene oxide) (not heated)                                             2-propanol                                                                          6.06E-02                                                                            6.34E-02                                                                           6.16E-02                                                                            6.05E-02                                                                           6.18E-02                                                                            6.38E-02                                    benzene                                                                             1.48E-01                                                                            1.47E-01                                                                           1.49E-01                                                                            1.50E-01                                                                           1.48E-01                                                                            1.47E-01                                    chloroform                                                                          1.42E-01                                                                            1.44E-01                                                                           1.43E-01                                                                            1.46E-01                                                                           1.43E-01                                                                            1.45E-01                                    cyclohexane                                                                         1.69E-02                                                                            1.63E-02                                                                           1.70E-02                                                                            1.66E-02                                                                           1.65E-02                                                                            1.65E-02                                    ethanol                                                                             5.66E-02                                                                            5.89E-02                                                                           5.77E-02                                                                            5.62E-02                                                                           5.74E-02                                                                            5.85E-02                                    hexane                                                                              1.27E-02                                                                            1.14E-02                                                                           1.26E-02                                                                            1.25E-02                                                                           1.22E-02                                                                            1.22E-02                                    methanol                                                                            2.69E-02                                                                            2.76E-02                                                                           2.74E-02                                                                            2.68E-02                                                                           2.72E-02                                                                            2.80E-02                                    toluene                                                                             5.36E-01                                                                            5.31E-01                                                                           5.32E-01                                                                            5.32E-01                                                                           5.34E-01                                                                            5.29E-01                                    Normalized STANDARD DEVIATION                                                 Sensor 4                                                                      poly(ethylene oxide) (not heated)                                             2-propanol                                                                          2.28E-03                                                                            1.24E-03                                                                           6.36E-04                                                                            2.06E-04                                                                           1.27E-04                                                                            2.26E-03                                    benzene                                                                             4.22E-03                                                                            4.61E-03                                                                           1.32E-03                                                                            1.94E-04                                                                           1.30E-03                                                                            3.51E-03                                    chloroform                                                                          6.22E-03                                                                            7.31E-03                                                                           2.71E-03                                                                            2.29E-03                                                                           4.33E-03                                                                            5.76E-03                                    cyclohexane                                                                         3.11E-04                                                                            1.05E-03                                                                           3.37E-06                                                                            1.51E-04                                                                           3.40E-04                                                                            3.55E-04                                    ethanol                                                                             2.53E-03                                                                            2.74E-04                                                                           4.01E-04                                                                            4.21E-04                                                                           5.31E-04                                                                            2.68E-03                                    hexane                                                                              2.48E-04                                                                            1.39E-03                                                                           6.69E-05                                                                            5.57E-05                                                                           2.66E-04                                                                            4.18E-04                                    methanol                                                                            4.06E-04                                                                            4.45E-04                                                                           4.64E-04                                                                            3.70E-04                                                                           2.29E-04                                                                            9.06E-04                                    toluene                                                                             7.97E-03                                                                            2.34E-03                                                                           3.23E-03                                                                            4.07E-03                                                                           1.62E-03                                                                            7.97E-03                                    Normalized averages                                                           Sensor 6                                                                      poly(ethylene oxide) (not heated)                                             2-propanol                                                                          5.53E-02                                                                            5.86E-02                                                                           5.65E-02                                                                            5.53E-02                                                                           5.69E-02                                                                            5.86E-02                                    benzene                                                                             1.53E-01                                                                            1.50E-01                                                                           1.53E-01                                                                            1.55E-01                                                                           1.53E-01                                                                            1.51E-01                                    chloroform                                                                          1.40E-01                                                                            1.42E-01                                                                           1.41E-01                                                                            1.43E-01                                                                           1.40E-01                                                                            1.43E-01                                    cyclohexane                                                                         1.75E-02                                                                            1.63E-02                                                                           1.76E-02                                                                            1.72E-02                                                                           1.70E-02                                                                            1.72E-02                                    ethanol                                                                             4.98E-02                                                                            5.28E-02                                                                           5.10E-02                                                                            4.97E-02                                                                           5.14E-02                                                                            5.24E-02                                    hexane                                                                              1.39E-02                                                                            1.16E-02                                                                           1.35E-02                                                                            1.35E-02                                                                           1.29E-02                                                                            1.32E-02                                    methanol                                                                            2.36E-02                                                                            2.46E-02                                                                           2.43E-02                                                                            2.36E-02                                                                           2.41E-02                                                                            2.49E-02                                    toluene                                                                             5.47E-01                                                                            5.44E-01                                                                           5.43E-01                                                                            5.43E-01                                                                           5.45E-01                                                                            5.40E-01                                    Normalized STANDARD DEVIATION                                                 Sensor 6                                                                      poly(ethylene oxide) (not heated)                                             2-propanol                                                                          2.36E-03                                                                            1.76E-03                                                                           6.15E-04                                                                            3.46E-04                                                                           2.65E-04                                                                            2.42E-03                                    benzene                                                                             4.47E-03                                                                            8.71E-03                                                                           1.36E-03                                                                            2.59E-04                                                                           1.63E-03                                                                            3.91E-03                                    chloroform                                                                          6.22E-03                                                                            9.18E-03                                                                           2.66E-03                                                                            2.20E-03                                                                           4.88E-03                                                                            5.88E-03                                    cyclohexane                                                                         3.90E-04                                                                            1.73E-03                                                                           1.23E-04                                                                            2.53E-04                                                                           4.51E-04                                                                            4.35E-04                                    ethanol                                                                             2.23E-03                                                                            7.86E-04                                                                           4.03E-04                                                                            2.16E-04                                                                           2.87E-04                                                                            2.47E-03                                    hexane                                                                              9.70E-05                                                                            2.27E-03                                                                           1.43E-04                                                                            1.06E-04                                                                           3.53E-04                                                                            3.45E-04                                    methanol                                                                            4.47E-04                                                                            7.32E-04                                                                           4.36E-04                                                                            2.54E-04                                                                           2.25E-04                                                                            9.89E-04                                    toluene                                                                             7.87E-03                                                                            8.31E-03                                                                           2.69E-03                                                                            4.53E-03                                                                           2.34E-03                                                                            8.03E-03                                    Normalized averages                                                           Sensor 9                                                                      poly(ethylene oxide) (not heated)                                             propanol                                                                            7.33E-02                                                                            8.62E-02                                                                           7.74E-02                                                                            7.45E-02                                                                           7.72E-02                                                                            9.14E-02                                    benzene                                                                             1.43E-01                                                                            1.42E-01                                                                           1.43E-01                                                                            1.44E-01                                                                           1.41E-01                                                                            1.38E-01                                    chloroform                                                                          1.50E-01                                                                            1.55E-01                                                                           1.52E-01                                                                            1.54E-01                                                                           1.55E-01                                                                            1.58E-01                                    cyclohexane                                                                         1.78E-02                                                                            2.23E-02                                                                           1.78E-02                                                                            1.72E-02                                                                           1.74E-02                                                                            1.78E-02                                    ethanol                                                                             6.89E-02                                                                            7.93E-02                                                                           6.97E-02                                                                            6.73E-02                                                                           7.28E-02                                                                            8.01E-02                                    hexane                                                                              1.24E-02                                                                            2.00E-02                                                                           1.26E-02                                                                            1.33E-02                                                                           1.25E-02                                                                            1.49E-02                                    methanol                                                                            3.42E-02                                                                            4.32E-02                                                                           3.39E-02                                                                            3.43E-02                                                                           3.78E-02                                                                            4.32E-02                                    toluene                                                                             5.01E-01                                                                            4.52E-01                                                                           4.93E-01                                                                            4.95E-01                                                                           4.86E-01                                                                            4.57E-01                                    Normalized STANDARD DEVIATION                                                 Sensor 9                                                                      poly(ethylene oxide) (not heated)                                             2-propanol                                                                          3.67E-03                                                                            3.73E-03                                                                           1.64E-03                                                                            6.77E-04                                                                           2.83E-03                                                                            1.00E-02                                    benzene                                                                             5.16E-03                                                                            1.20E-02                                                                           3.16E-03                                                                            2.47E-03                                                                           9.94E-04                                                                            2.41E-03                                    chloroform                                                                          4.69E-03                                                                            3.34E-03                                                                           1.32E-03                                                                            1.44E-03                                                                           3.40E-03                                                                            1.08E-02                                    cyclohexane                                                                         1.15E-03                                                                            4.09E-03                                                                           7.89E-04                                                                            2.21E-03                                                                           2.41E-04                                                                            1.01E-03                                    ethanol                                                                             3.94E-03                                                                            5.11E-03                                                                           3.86E-04                                                                            1.37E-03                                                                           4.78E-04                                                                            4.93E-03                                    hexane                                                                              1.90E-03                                                                            6.97E-03                                                                           1.12E-03                                                                            1.10E-03                                                                           4.08E-03                                                                            4.35E-03                                    methanol                                                                            1.72E-03                                                                            2.46E-03                                                                           6.74E-04                                                                            1.25E-03                                                                           2.04E-03                                                                            3.49E-03                                    toluene                                                                             9.70E-03                                                                            8.90E-03                                                                           1.91E-03                                                                            6.27E-03                                                                           1.07E-02                                                                            8.81E-03                                    __________________________________________________________________________    22° C.                                                                             22° C.                                                                      23° C.                                                                       44° C.                                                                      47° C.                                                                       57° C.                               __________________________________________________________________________    Normalized averages                                                           Sensor 15                                                                     poly(ethylene oxide)                                                          2-propanol                                                                          1.18E-01                                                                            1.22E-01                                                                           1.22E-01                                                                            9.01E-02                                                                           1.14E-01                                                                            9.98E-02                                    benzene                                                                             1.21E-01                                                                            1.27E-01                                                                           1.20E-01                                                                            1.14E-01                                                                           1.43E-01                                                                            1.06E-01                                    chloroform                                                                          1.52E-01                                                                            1.52E-01                                                                           1.51E-01                                                                            1.37E-01                                                                           1.48E-01                                                                            1.15E-01                                    cyclohexane                                                                         2.39E-02                                                                            3.53E-02                                                                           2.62E-02                                                                            1.69E-02                                                                           4.46E-02                                                                            5.62E-02                                    ethanol                                                                             1.09E-01                                                                            1.15E-01                                                                           1.11E-01                                                                            9.78E-02                                                                           9.05E-02                                                                            1.14E-01                                    hexane                                                                              1.84E-02                                                                            2.83E-02                                                                           1.92E-02                                                                            3.02E-02                                                                           3.23E-02                                                                            2.34E-02                                    methanol                                                                            5.54E-02                                                                            6.39E-02                                                                           5.71E-02                                                                            5.17E-02                                                                           4.85E-02                                                                            7.93E-02                                    toluene                                                                             4.02E-01                                                                            3.56E-01                                                                           3.93E-01                                                                            4.63E-01                                                                           3.79E-01                                                                            4.06E-01                                    Normalized STANDARD DEVIATION                                                 Sensor 15                                                                     poly(ethylene oxide)                                                          2-propanol                                                                          3.12E-03                                                                            1.56E-03                                                                           1.68E-03                                                                            3.25E-02                                                                           7.71E-03                                                                            3.05E-02                                    benzene                                                                             9.84E-04                                                                            1.82E-02                                                                           2.67E-03                                                                            4.78E-02                                                                           1.81E-02                                                                            8.01E-02                                    chloroform                                                                          4.71E-03                                                                            7.72E-03                                                                           1.23E-03                                                                            2.09E-02                                                                           1.10E-02                                                                            3.47E-02                                    cyclohexane                                                                         1.14E-03                                                                            4.95E-03                                                                           5.91E-03                                                                            6.16E-03                                                                           2.33E-02                                                                            7.55E-02                                    ethanol                                                                             3.63E-03                                                                            6.63E-03                                                                           6.06E-03                                                                            1.02E-02                                                                           1.08E-02                                                                            6.71E-02                                    hexane                                                                              3.46E-03                                                                            3.94E-03                                                                           5.55E-04                                                                            3.68E-02                                                                           1.44E-02                                                                            1.16E-02                                    methanol                                                                            3.27E-03                                                                            1.48E-03                                                                           5.57E-03                                                                            9.42E-03                                                                           1.56E-02                                                                            4.16E-02                                    toluene                                                                             5.22E-03                                                                            1.49E-02                                                                           4.32E-03                                                                            9.75E-03                                                                           1.71E-02                                                                            7.70E-02                                    __________________________________________________________________________    22° C.                                                                             22° C.                                                                      23° C.                                                                       33° C.                                                                      33° C.                                                                       37° C.                               __________________________________________________________________________    Normalized averages                                                           Sensor 16                                                                     poly(ethylene oxide)                                                          2-propanol                                                                          6.88E-02                                                                            7.87E-02                                                                           7.22E-02                                                                            6.11E-02                                                                           6.60E-02                                                                            6.75E-02                                    benzene                                                                             1.45E-01                                                                            1.42E-01                                                                           1.46E-01                                                                            1.51E-01                                                                           1.50E-01                                                                            1.47E-01                                    chloroform                                                                          1.48E-01                                                                            1.52E-01                                                                           1.51E-01                                                                            1.39E-01                                                                           1.41E-01                                                                            1.38E-01                                    cyclohexane                                                                         1.53E-02                                                                            1.73E-02                                                                           1.55E-02                                                                            1.72E-02                                                                           1.78E-02                                                                            2.06E-02                                    ethanol                                                                             6.26E-02                                                                            7.29E-02                                                                           6.54E-02                                                                            5.69E-02                                                                           6.03E-02                                                                            6.38E-02                                    hexane                                                                              9.63E-03                                                                            1.33E-02                                                                           1.07E-02                                                                            1.16E-02                                                                           1.28E-02                                                                            1.50E-02                                    methanol                                                                            3.00E-02                                                                            3.68E-02                                                                           3.11E-02                                                                            2.91E-02                                                                           3.09E-02                                                                            3.60E-02                                    toluene                                                                             5.21E-01                                                                            4.86E-01                                                                           5.08E-01                                                                            5.33E-01                                                                           5.21E-01                                                                            5.12E-01                                    Normalized STANDARD DEVIATION                                                 Sensor 16                                                                     poly(ethylene oxide)                                                          2-propanol                                                                          2.68E-03                                                                            2.04E-03                                                                           6.52E-04                                                                            1.39E-03                                                                           8.06E-04                                                                            2.45E-03                                    benzene                                                                             4.83E-03                                                                            4.29E-03                                                                           1.12E-03                                                                            1.31E-03                                                                           2.29E-03                                                                            5.57E-03                                    chloroform                                                                          7.22E-03                                                                            2.08E-03                                                                           2.94E-03                                                                            1.55E-03                                                                           2.29E-03                                                                            6.97E-03                                    cyclohexane                                                                         1.60E-04                                                                            1.86E-03                                                                           2.13E-04                                                                            1.39E-03                                                                           1.51E-03                                                                            8.98E-04                                    ethanol                                                                             2.89E-03                                                                            2.78E-03                                                                           3.73E-04                                                                            9.62E-04                                                                           4.09E-04                                                                            2.02E-03                                    hexane                                                                              4.86E-04                                                                            2.03E-03                                                                           1.21E-04                                                                            9.89E-04                                                                           8.28E-04                                                                            1.62E-03                                    methanol                                                                            4.58E-04                                                                            1.31E-03                                                                           7.76E-04                                                                            3.98E-04                                                                           1.58E-03                                                                            8.50E-04                                    toluene                                                                             9.39E-03                                                                            6.32E-03                                                                           4.35E-03                                                                            7.19E-03                                                                           6.42E-03                                                                            5.87E-03                                    __________________________________________________________________________    22° C.                                                                             22° C.                                                                      23° C.                                                                       22° C.                                                                      22° C.                                                                       22° C.                               __________________________________________________________________________    Normalized averages                                                           Sensor 5                                                                      poly(4-vinyl phenol) (not heated)                                             2-propanol                                                                          9.08E-02                                                                            5.67E-02                                                                           7.83E-02                                                                            9.08E-02                                                                           7.68E-02                                                                            5.63E-02                                    benzene                                                                             1.40E-02                                                                            1.22E-02                                                                           1.46E-02                                                                            1.49E-02                                                                           1.41E-02                                                                            1.45E-02                                    chloroform                                                                          2.73E-02                                                                            2.26E-02                                                                           2.76E-02                                                                            3.00E-02                                                                           2.82E-02                                                                            2.19E-02                                    cyclohexane                                                                         3.62E-03                                                                            2.77E-03                                                                           3.05E-03                                                                            2.17E-03                                                                           2.79E-03                                                                            3.72E-03                                    ethanol                                                                             4.81E-01                                                                            4.71E-01                                                                           4.81E-01                                                                            4.75E-01                                                                           4.77E-01                                                                            4.68E-01                                    hexane                                                                              3.65E-03                                                                            3.72E-03                                                                           3.67E-03                                                                            4.97E-03                                                                           3.45E-03                                                                            3.92E-03                                    methanol                                                                            3.50E-01                                                                            4.06E-01                                                                           3.62E-01                                                                            3.51E-01                                                                           3.71E-01                                                                            4.08E-01                                    toluene                                                                             2.98E-02                                                                            2.49E-02                                                                           2.98E-02                                                                            3.09E-02                                                                           2.73E-02                                                                            2.46E-02                                    Normalized STANDARD DEVIATION                                                 Sensor 5                                                                      poly(4-vinyl phenol) (not heated)                                             2-propanol                                                                          3.47E-03                                                                            2.54E-03                                                                           1.62E-03                                                                            2.13E-03                                                                           2.63E-03                                                                            3.99E-03                                    benzene                                                                             5.06E-04                                                                            1.03E-03                                                                           1.30E-03                                                                            1.14E-03                                                                           6.17E-04                                                                            8.88E-04                                    chloroform                                                                          1.85E-03                                                                            1.25E-03                                                                           6.47E-04                                                                            1.66E-03                                                                           7.39E-04                                                                            1.63E-03                                    cyclohexane                                                                         4.16E-04                                                                            1.17E-03                                                                           4.08E-04                                                                            1.46E-03                                                                           5.21E-04                                                                            2.23E-03                                    ethanol                                                                             1.55E-02                                                                            1.84E-02                                                                           4.28E-03                                                                            1.48E-02                                                                           1.05E-02                                                                            2.24E-02                                    hexane                                                                              9.72E-04                                                                            3.32E-04                                                                           9.08E-04                                                                            9.08E-04                                                                           1.21E-04                                                                            9.83E-04                                    methanol                                                                            1.46E-03                                                                            6.28E-03                                                                           4.61E-03                                                                            4.63E-03                                                                           3.12E-03                                                                            1.44E-02                                    toluene                                                                             1.39E-03                                                                            1.77E-03                                                                           2.80E-03                                                                            2.63E-03                                                                           1.51E-03                                                                            2.14E-04                                    __________________________________________________________________________

The average ΔR/R values for exposures to the eight solvents are listedin Table 6 and are presented graphically in the various panels of FIG.11. It can be seen that as the temperature increases, the over-allresponse of a given sensor decreases (in accord with the expecteddecrease in gas/polymer partition coefficient with increasingtemperature). In addition, the error bars became larger at the highertemperatures as the signal became smaller relative to the noise.

Because of the differences in over-all height of the responses, thedifferences in response patterns between the room temperature data andthe elevated temperature data are difficult to visualize from the plotsof the absolute ΔR/R response of the sensor array. The patterns were,therefore, normalized by adding all the responses by a sensor to theeight solvents and dividing this sum by each response to a solvent. Thisnormalization process highlights the differences in the pattern ofresponses at different temperatures rather than the over-all heightdifferences. Table 7 and the various panels of FIG. 12 show thenormalized patterns for the sensors.

In general, the fingerprints for the various vapors were similar at thedifferent temperatures, but the magnitudes of the absolute responseswere quite different. This implies that the characteristic "fingerprint"pattern of the analytes chosen in this study, for the sensor filmsinvestigated, were essentially invariant over the sensor materials andtemperature range probed herein. This will be beneficial in manyapplications, (quality control of foodstuffs, process monitoring, etc.)in that identification of a vapor or (of its constancy in composition)over various exposures to the array, and classification of the vaportype in any given exposure, using the response pattern of the sensoroutput signals will not significantly depend on the temperature of thesensor array or of the analyte.

Quantitative analysis of the concentration of the analyte, would,however, require knowledge of the sensor temperature. This effect can beused to advantage in other potential applications of these sensorarrays, since lowering the temperature of an array element will ingeneral increase the signal and therefore increase the sensitivity ofthe sensor to the desired analyte. Thus, a plurality of compositionallyidentical sensors, each held at a different temperature during ameasurement period, could be used to produce signals above a thresholdvalue at different concentrations of the vapor, thereby aiding inquantifying various ranges of the vapor concentration while stillmaintaining a linear concentration vs. vapor concentration response (inthe small swelling regime) for an individual sensor. In addition, thedifferential response of compositionally identical sensors at varioustemperatures can be used to provide classification and identificationinformation as the basis for the output signature of the sensor array.In general, combinations of compositionally different sensors at aplurality of different temperatures will produce a more desirable, moreinformation-rich, data set from a given set of sensor materials thanmeasurements at one fixed temperature, and can such arrays can thereforebe usefully exploited for the purpose of detection, identification, andquantification, of a particular analyte. Such detection, identificationand quantitiation may also be performed by employing a single resistorat a plurality of different temperatures.

All publications and patent applications cited in this specification areherein incorporated by reference as if each individual publication orpatent application were specifically and individually indicated to beincorporated by reference. Although the foregoing invention has beendescribed in some detail by way of illustration and example for purposesof clarity of understanding, it will be readily apparent to those ofordinary skill in the art in light of the teachings of this inventionthat certain changes and modifications may be made thereto withoutdeparting from the spirit or scope of the appended claims.

What is claimed is:
 1. A sensor for detecting the presence of a chemicalanalyte in a fluid, said sensor comprising a chemically sensitiveresistor electrically connected to an electrical measuring apparatus andbeing in thermal communication with a temperature control apparatus,said chemically sensitive resistor comprising:a mixture of nonconductiveorganic polymer and a conductive material compositionally different thansaid nonconductive organic polymer, wherein said resistor provides anelectrical path through said mixture of nonconductive organic polymerand said conductive material, and an electrical resistance R_(m) attemperature T_(m) when contacted with a fluid comprising said chemicalanalyte, where m is an integer greater than
 1. 2. The sensor accordingto claim 1 wherein said conductive material is an inorganic conductor.3. The sensor according to claim 1 wherein said conductive material isan organic conductor.
 4. The sensor according to claim 1 wherein saidsensor comprises an array of two or more of said chemically sensitiveresistors, wherein each of said chemically sensitive resistors iselectrically connected to an electrical measuring apparatus and is inthermal communication with a temperature control apparatus.
 5. Thesensor according to claim 4 wherein said nonconductive organic polymerin one of said chemically sensitive resistors in said array is differentfrom said nonconductive organic polymer in at least one other chemicallysensitive resistor in said array.
 6. The sensor according to claim 4wherein said conductive material in one chemically sensitive resistor insaid array is different from said conductive material in at least oneother chemically sensitive resistor in said array.
 7. The sensoraccording to claim 4 wherein each of said chemically sensitive resistorsin said array are compositionally the same.
 8. A sensor for detectingthe presence of a chemical analyte in a fluid, said sensor comprising achemically sensitive resistor electrically connected to an electricalmeasuring apparatus, said chemically sensitive resistor comprising:amixture of nonconductive organic polymer and a conductive materialcompositionally different than said nonconductive organic polymer,wherein said resistor provides an electrical path through said mixtureof nonconductive organic polymer and said conductive material, and anelectrical impedance Z_(m) at frequency ω_(m) when contacted with afluid comprising said chemical analyte, where m is an integer greaterthan 1 and ω_(m) does not equal
 0. 9. The sensor according to claim 8wherein said conductive material is an inorganic conductor.
 10. Thesensor according to claim 8 wherein said conductive material is anorganic conductor.
 11. The sensor according to claim 8 wherein saidsensor comprises an array of two or more of said chemically sensitiveresistors, wherein each of said chemically sensitive resistors iselectrically connected to an electrical measuring apparatus.
 12. Thesensor according to claim 11 wherein said nonconductive organic polymerin one of said chemically sensitive resistors in said array is differentfrom said nonconductive organic polymer in at least one other chemicallysensitive resistor in said array.
 13. The sensor according to claim 11wherein said conductive material in one chemically sensitive resistor insaid array is different from said conductive material in at least oneother chemically sensitive resistor in said array.
 14. The sensoraccording to claim 11 wherein each of said chemically sensitiveresistors in said array are compositionally the same.
 15. A sensor fordetecting the presence of a chemical analyte in a fluid, said sensorcomprising a chemically sensitive resistor electrically connected to anelectrical measuring apparatus and being in thermal communication with atemperature control apparatus, said chemically sensitive resistorcomprising:a mixture of nonconductive organic polymer and a conductivematerial compositionally different than said nonconductive organicpolymer, wherein said resistor provides an electrical path through saidmixture of nonconductive organic polymer and said conductive material,and an electrical impedance Z_(m),n at frequency ω_(m) and temperatureT_(n) when contacted with a fluid comprising said chemical analyte,where m and/or n is an integer greater than
 1. 16. The sensor accordingto claim 15, wherein m and n are integers greater than
 1. 17. The sensoraccording to claim 15 wherein said conductive material is an inorganicconductor.
 18. The sensor according to claim 15 wherein said conductivematerial is an organic conductor.
 19. The sensor according to claim 15wherein said sensor comprises an array of two or more of said chemicallysensitive resistors, wherein each of said chemically sensitive resistorsis electrically connected to an electrical measuring apparatus and is inthermal communication with a temperature control apparatus.
 20. Thesensor according to claim 19 wherein said nonconductive organic polymerin one of said chemically sensitive resistor in said array is differentfrom said nonconductive organic polymer in at least one other chemicallysensitive resistor in said array.
 21. The sensor according to claim 19wherein said conductive material in one chemically sensitive resistor insaid array is different from said conductive material in at least oneother chemically sensitive resistor in said array.
 22. The sensoraccording to claim 19 wherein each of said chemically sensitiveresistors in said array are compositionally the same.
 23. A method fordetecting the presence of a chemical analyte in a fluid, said methodcomprising:contacting a sensor with a fluid comprising said chemicalanalyte, said sensor comprising a chemically sensitive resistorelectrically connected to an electrical measuring apparatus and being inthermal communication with a temperature control apparatus, wherein saidchemically sensitive resistor comprises a mixture of nonconductiveorganic polymer and a conductive material compositionally different thansaid nonconductive organic polymer and provides an electrical paththrough said mixture of nonconductive organic polymer and saidconductive material, and measuring the electrical resistance R_(m) attemperature T_(m), where m is an integer greater than 1,wherein saidelectrical resistance R_(m) at temperature T_(m) is indicative of thepresence or absence of said chemical analyte in said fluid.
 24. A methodfor detecting the presence of a chemical analyte in a fluid, said methodcomprising:contacting a sensor with a fluid comprising said chemicalanalyte, said sensor comprising a chemically sensitive resistorelectrically connected to an electrical measuring apparatus, whereinsaid chemically sensitive resistor comprises a mixture of nonconductiveorganic polymer and a conductive material compositionally different thansaid nonconductive organic polymer and provides an electrical paththrough said mixture of nonconductive organic polymer and saidconductive material, and measuring the electrical impedance Z_(m) atfrequency ω_(m), where m is an integer greater than 1 and ω_(m) does notequal 0,wherein said electrical impedance Z_(m) at frequency ω_(m) isindicative of the presence or absence of said chemical analyte in saidfluid.
 25. A method for detecting the presence of a chemical analyte ina fluid, said method comprising:contacting a sensor with a fluidcomprising said chemical analyte, said sensor comprising a chemicallysensitive resistor electrically connected to an electrical measuringapparatus and being in thermal communication with a temperature controlapparatus, wherein said chemically sensitive resistor comprises amixture of nonconductive organic polymer and a conductive materialcompositionally different than said nonconductive organic polymer andprovides an electrical path through said mixture of nonconductiveorganic polymer and said conductive material, and measuring theelectrical impedance Z_(m),n at frequency ω_(m) and temperature T_(n),where m and/or n is an integer greater than 1,wherein said electricalimpedance Z_(m),n at frequency ω_(m) and at temperature T_(n) isindicative of the presence or absence of said chemical analyte in saidfluid.